[math] Remove more bare variable support

Prior to this fix, a "bare variable" in math like 'x + 1' would be
looked up in the environment, i.e. equivalent to '$x + 1'. This appears
to have been done for performance. However this breaks the orthogonality
of fish; performance is not a sufficient justification to give math this
level of built-in power, especially because the performance of math is
not a bottleneck. The implementation is also ugly.

Remove this feature so that variables must be prefixed with the dollar
sign and undergo normal variable expansion. Reading 'git grep' output
does not show any uses of this in fish functions or completions.

Also added to changelog.

Fixes #4393

tests/math.out

@@ -17,7 +17,7 @@
 -8
 
 ####################
-# Validate how bare variables in an epxression are handled
+# Validate how variables in an expression are handled
 1
 2
 -4