diff: remove DIFF_OPT_CLR macro

diff: remove DIFF_OPT_CLR macro

Remove the `DIFF_OPT_CLR` macro and instead set the flags directly.
This conversion is done using the following semantic patch:

	@@
	expression E;
	identifier fld;
	@@
	- DIFF_OPT_CLR(&E, fld)
	+ E.flags.fld = 0

	@@
	type T;
	T *ptr;
	identifier fld;
	@@
	- DIFF_OPT_CLR(ptr, fld)
	+ ptr->flags.fld = 0

Signed-off-by: Brandon Williams <redacted>
Signed-off-by: Junio C Hamano <redacted>