Remove now-unused Modula-2 evaluator code

Remove now-unused Modula-2 evaluator code

Now that the Modula-2 parser has switched to the new style, there is
no need for the old Modula-2 evaluation code.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* m2-lang.h (class m2_language) <expresssion_ops,
	exp_descriptor_modula2>: Remove.
	* m2-lang.c (evaluate_subexp_modula2)
	(m2_language::exp_descriptor_modula2): Remove.