c++: remove COMPOUND_EXPR_OVERLOADED flag

This flag is never set because non-dependent COMPOUND_EXPRs that resolve
to an overload are expressed as a CALL_EXPR at template definition time
(in build_x_compound_expr) ever since r6-5772.

gcc/cp/ChangeLog:

	* cp-tree.h (COMPOUND_EXPR_OVERLOADED): Remove.
	* pt.c (build_non_dependent_expr): Don't inspect the flag.
	* tree.c (build_min_non_dep): Don't set the flag.