gh-40656: remove deprecated method in matroid.pyx
    
This was deprecated in #36394

### 📝 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: #40656
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/matroids/matroid.pxd

@@ -148,7 +148,6 @@ cdef class Matroid(SageObject):
     cpdef minor(self, contractions=*, deletions=*)
     cpdef contract(self, X)
     cpdef delete(self, X)
-    cpdef _backslash_(self, X)
     cpdef dual(self)
     cpdef truncation(self)
     cpdef has_minor(self, N, bint certificate=*)

src/sage/matroids/matroid.pyx

@@ -348,7 +348,7 @@ from itertools import combinations, product
 from sage.matrix.constructor import matrix
 from sage.misc.lazy_import import LazyImport
 from sage.misc.prandom import shuffle
-from sage.misc.superseded import deprecation, deprecated_function_alias
+from sage.misc.superseded import deprecated_function_alias
 from sage.rings.integer_ring import ZZ
 from sage.structure.richcmp cimport rich_to_bool, richcmp
 from sage.structure.sage_object cimport SageObject
@@ -4217,9 +4217,6 @@ cdef class Matroid(SageObject):
         one. It can be shown that the resulting matroid does not depend on the
         order of the deletions.
 
-        DEPRECATED: Sage supports the shortcut notation ``M \ X`` for
-        ``M.delete(X)``.
-
         INPUT:
 
         - ``X`` -- either a single element of the groundset, or a collection
@@ -4268,23 +4265,6 @@ cdef class Matroid(SageObject):
         """
         return self.minor(deletions=X)
 
-    cpdef _backslash_(self, X):
-        r"""
-        Shorthand for ``self.delete(X)``.
-
-        Deprecated.
-
-        EXAMPLES::
-
-            sage: M = matroids.CompleteGraphic(4)                                       # needs sage.graphs
-            sage: M.delete(1) == M \ 1  # indirect doctest                              # needs sage.graphs
-            doctest:...: DeprecationWarning: the backslash operator has been deprecated; use M.delete(X) instead
-            See https://github.com/sagemath/sage/issues/36394 for details.
-            True
-        """
-        deprecation(36394, 'the backslash operator has been deprecated; use M.delete(X) instead')
-        return self.delete(X)
-
     cpdef dual(self):
         r"""
         Return the dual of the matroid.