MIPS: Makefile: Remove "ifdef need-compiler" for Kbuild.platforms

After commit 13ceb48b ("MIPS: Loongson2ef: Remove unnecessary
{as,cc}-option calls"), no need to use "ifdef need-compiler" for
Kbuild.platforms, because the cause of the build issue mentioned
in commit 0706f74f ("MIPS: fix *-pkg builds for loongson2ef
platform") has been disappeared, so just remove it.

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>