gh-40659: remove 2 deprecated functions in symbolic/
    
as deprecated in #35505

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
    
URL: #40659
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/symbolic/expression.pyx

@@ -408,76 +408,6 @@ include "pynac_impl.pxi"
 from sage.symbolic.symbols import symbol_table, register_symbol  # used to be defined in pynac_impl
 
 
-cpdef bint is_SymbolicEquation(x) noexcept:
-    """
-    Return ``True`` if ``x`` is a symbolic equation.
-
-    This function is deprecated.
-
-    EXAMPLES:
-
-    The following two examples are symbolic equations::
-
-        sage: from sage.symbolic.expression import is_SymbolicEquation
-        sage: is_SymbolicEquation(sin(x) == x)
-        doctest:warning...
-        DeprecationWarning: is_SymbolicEquation is deprecated; use
-        'isinstance(x, sage.structure.element.Expression) and x.is_relational()' instead
-        See https://github.com/sagemath/sage/issues/35505 for details.
-        True
-        sage: is_SymbolicEquation(sin(x) < x)
-        True
-        sage: is_SymbolicEquation(x)
-        False
-
-    This is not, since ``2==3`` evaluates to the boolean
-    ``False``::
-
-        sage: is_SymbolicEquation(2 == 3)
-        False
-
-    However here since both 2 and 3 are coerced to be symbolic, we
-    obtain a symbolic equation::
-
-        sage: is_SymbolicEquation(SR(2) == SR(3))
-        True
-    """
-    from sage.misc.superseded import deprecation
-    deprecation(35505,
-                "is_SymbolicEquation is deprecated; use "
-                "'isinstance(x, sage.structure.element.Expression) and x.is_relational()' instead")
-    return isinstance(x, Expression) and is_a_relational((<Expression>x)._gobj)
-
-
-# Defined here but exported by sage.symbolic.ring
-cpdef bint _is_SymbolicVariable(x) noexcept:
-    """
-    Return ``True`` if ``x`` is a variable.
-
-    EXAMPLES::
-
-        sage: from sage.symbolic.ring import is_SymbolicVariable
-        sage: is_SymbolicVariable(x)
-        doctest:warning...
-        DeprecationWarning: is_SymbolicVariable is deprecated; use
-        'isinstance(x, sage.structure.element.Expression) and x.is_symbol()' instead
-        See https://github.com/sagemath/sage/issues/35505 for details.
-        True
-        sage: is_SymbolicVariable(x + 2)
-        False
-
-    TESTS::
-
-        sage: ZZ['x']
-        Univariate Polynomial Ring in x over Integer Ring
-    """
-    from sage.misc.superseded import deprecation
-    deprecation(35505,
-                "is_SymbolicVariable is deprecated; use "
-                "'isinstance(x, sage.structure.element.Expression) and x.is_symbol()' instead")
-    return isinstance(x, Expression) and is_a_symbol((<Expression>x)._gobj)
-
-
 def _dict_update_check_duplicate(dict d1, dict d2):
     r"""
     Merge the dictionary ``d2`` into ``d1`` and check for duplicates.
@@ -4540,7 +4470,7 @@ cdef class Expression(Expression_abc):
             sage: f.derivative(2)
             x |--> 6*x - sin(x)
 
-        Some expressions can't be cleanly differentiated by the
+        Some expressions cannot be cleanly differentiated by the
         chain rule::
 
             sage: _ = var('x', domain='real')

src/sage/symbolic/ring.pyx

@@ -52,12 +52,6 @@ from sage.structure.coerce cimport is_numpy_type
 import sage.rings.abc
 from sage.rings.integer_ring import ZZ
 
-# is_SymbolicVariable used to be defined here; re-export it here lazily
-cpdef bint is_SymbolicVariable(x):
-    from sage.symbolic.expression import _is_SymbolicVariable
-
-    return _is_SymbolicVariable(x)
-
 import keyword
 import operator