Remove now-unused Rust evaluator code

Remove now-unused Rust evaluator code

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

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

	* rust-lang.h (class rust_language) <expression_ops,
	exp_descriptor_tab>: Remove.
	* rust-lang.c (rust_evaluate_funcall): Remove.
	(rust_range, rust_subscript, eval_op_rust_complement): Don't use
	EVAL_SKIP.
	(rust_evaluate_subexp): Remove.
	(rust_aggregate_operation::evaluate): Don't use EVAL_SKIP.
	(rust_operator_length, rust_dump_subexp_body, rust_print_subexp)
	(rust_operator_check, rust_language::exp_descriptor_tab): Remove.