MIPS: SGI-IP22: Remove unnecessary check of GCC option

According to Documentation/process/changes.rst, the minimal version of GCC
is 5.1, and -mr10k-cache-barrier=store is supported with GCC 5.1 [1], just
remove the unnecessary check to fix the build error [2]:

  arch/mips/sgi-ip22/Platform:28: *** gcc doesn't support needed option -mr10k-cache-barrier=store.  Stop.

[1] https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gcc/MIPS-Options.html
[2] https://github.com/ClangBuiltLinux/linux/issues/1543



Reported-by: Ryutaroh Matsumoto <ryutaroh@ict.e.titech.ac.jp>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>