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.