Revert "'is_field' method added returning 'True'"

This reverts commit aa7dbaf.

Author: Michael Jung

src/sage/manifolds/scalarfield.py

@@ -1203,6 +1203,28 @@ def is_trivial_zero(self):
             return True
         return all(func.is_trivial_zero() for func in self._express.values())
 
+    def is_unit(self):
+        r"""
+        Return ``True`` iff ``self`` is not trivially zero in at least one of
+        the given expressions since most scalar fields are invertible and a
+        complete computation would take too much time.
+
+        EXAMPLES::
+
+            sage: M = Manifold(2, 'M', structure='top')
+            sage: one = M.scalar_field_algebra().one()
+            sage: one.is_unit()
+            True
+            sage: zero = M.scalar_field_algebra().zero()
+            sage: zero.is_unit()
+            False
+
+        """
+        if self._is_zero:
+            return False
+        return not any(func.is_trivial_zero()
+                       for func in self._express.values())
+
     def __eq__(self, other):
         r"""
         Comparison (equality) operator.

src/sage/manifolds/scalarfield_algebra.py

@@ -616,18 +616,3 @@ def one(self):
         return self.element_class(self,
                                   coord_expression=coord_express,
                                   name='1', latex_name='1')
-
-    def is_field(self, proof=True):
-        """
-        Return ``True``, since the algebra of scalar fields is (for the most
-        part) a field.
-
-        EXAMPLES::
-
-            sage: M = Manifold(2, 'M')
-            sage: A = M.scalar_field_algebra()
-            sage: A.is_field()
-            True
-
-        """
-        return True