Remove obsolete SpEL expression grammar file

The antlr-based SpEL expression grammar file (SpringExpressions.g) was
introduced during initial development and prototyping of the Spring
Expression language; however, it was quickly abandoned in favor of a
handcrafted implementation. Consequently, it has become obsolete over
time and has never been actively maintained.

This commit therefore removes the obsolete SpEL expression grammar file.

Closes gh-28425

Author: sbrannen