nios2: Remove custom instruction warnings

nios2: Remove custom instruction warnings

Do not warn if custom instructions are not used due to missing
optimization flags.  This prevents build errors with -Werror which
cannot be disabled via a dedicated warning option.

One reason to remove these warnings is to enable a multilib for the
"Nios II Floating Point Hardware 2 Component".  For example, the
libatomic target library in GCC is built with -Werror and the warnings
removed by this patch resulted in errors like:

cc1: error: switch '-mcustom-fmins' has no effect unless '-ffinite-math-only' is specified [-Werror]
cc1: error: switch '-mcustom-fmaxs' has no effect unless '-ffinite-math-only' is specified [-Werror]
cc1: error: switch '-mcustom-round' has no effect unless '-fno-math-errno' is specified [-Werror]

gcc/

	* config/nios2/nios2.c (nios2_custom_check_insns): Remove
	custom instruction warnings.