d: Revert "Come up with startswith function."

This reverts changes to the DMD front-end in commit
6ba3079d.

Changes were incorrectly committed directly to the GCC repo instead of
the master repository.

gcc/d/ChangeLog:

	* dmd/dinterpret.c (evaluateIfBuiltin): Revert last change.
	* dmd/dmangle.c: Likewise.
	* dmd/hdrgen.c: Likewise.
	* dmd/identifier.c (Identifier::toHChars2): Likewise.