MIPS: MIPSsim: Remove the MIPSsim platform.

The MIPSsim platform is no longer supported or used.

[ralf@linux-mips.org: Also remove mipssim from arch/mips/Kbuild.platforms
and delete arch/mips/include/asm/mach-mipssim/*.]

Signed-off-by: Steven J. Hill <sjhill@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4350/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>