Remove BINOP_END

Remove BINOP_END

BINOP_END is used only as a "meaningless" value in various tables.
This patch changes these to use OP_NULL instead, and removes
BINOP_END.

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

	* std-operator.def (BINOP_END): Remove.
	* p-exp.y (tokentab3, tokentab2): Use OP_NULL, not BINOP_END.
	* go-exp.y (tokentab2): Use OP_NULL, not BINOP_END.
	* f-exp.y (dot_ops, f77_keywords): Use OP_NULL, not BINOP_END.
	* d-exp.y (tokentab2, ident_tokens): Use OP_NULL, not BINOP_END.
	* c-exp.y (tokentab3, tokentab2, ident_tokens): Use OP_NULL, not
	BINOP_END.