compiler-warnings: Drop -Wno-sign-comare

It's not part of -Wall, and we're not explicitly turning it on here,
so there's no point in turning it off, since it's not on.

Additionally, if a given module did want it on, it's clearer if
the compiler flags don't have -Wno-sign-compare -Wsign-compare.

https://bugzilla.gnome.org/show_bug.cgi?id=688192