Remove now-unused C evaluator code

Remove now-unused C evaluator code

Now that the C parser has switched to the new style, there is no need
for the old C evaluation code.  This affects some other languages that
were relying on the C code.

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

	* go-lang.c (go_language::expression_ops): Don't declare.
	* go-lang.h (class go_language) <expression_ops>: Remove.
	* opencl-lang.c (evaluate_subexp_opencl, exp_descriptor_opencl):
	Remove.
	(class opencl_language) <expression_ops>: Remove.
	* d-lang.c (class d_language) <expression_ops>: Remove.
	* c-lang.h (evaluate_subexp_c, exp_descriptor_c): Don't declare.
	* c-lang.c (evaluate_subexp_c, exp_descriptor_c): Remove.
	(class c_language, class cplus_language, class asm_language)
	(class minimal_language) <expression_ops>: Remove.