diff: remove unused macros `DIFF_FLAG_*`

In commit 9be638ec (git_diff_generated: abstract generated diffs,
2016-04-19), the code for generated diffs was moved out of the generic
"diff.c" and instead into its own module. During that conversion, it was
forgotten to remove the macros `DIFF_FLAG_IS_SET`, `DIFF_FLAG_ISNT_SET`
and `DIFF_FLAG_SET`, which are now only used in "diff_generated.c".

Remove those macros now.

src/diff.c

@@ -13,13 +13,6 @@
 #include "commit.h"
 #include "index.h"
 
-#define DIFF_FLAG_IS_SET(DIFF,FLAG) \
-	(((DIFF)->opts.flags & (FLAG)) != 0)
-#define DIFF_FLAG_ISNT_SET(DIFF,FLAG) \
-	(((DIFF)->opts.flags & (FLAG)) == 0)
-#define DIFF_FLAG_SET(DIFF,FLAG,VAL) (DIFF)->opts.flags = \
-	(VAL) ? ((DIFF)->opts.flags | (FLAG)) : ((DIFF)->opts.flags & ~(VAL))
-
 struct patch_id_args {
 	git_hash_ctx ctx;
 	git_oid result;