rust: build failure after NON_DEPENDENT_EXPR removal [PR111899]

This patch removes stray NON_DEPENDENT_EXPR checks following the removal
of this tree code from the C++ FE.  (Since this restores the build I
supppose it means the Rust FE never creates NON_DEPENDENT_EXPR trees in
the first place, so no further analysis is needed.)

	PR rust/111899

gcc/rust/ChangeLog:

	* backend/rust-constexpr.cc (potential_constant_expression_1):
	Remove NON_DEPENDENT_EXPR handling.
	* backend/rust-tree.cc (mark_exp_read): Likewise.
	(mark_use): Likewise.
	(lvalue_kind): Likewise.