MIPS: Remove unused R8000 CPU support

Our R8000 CPU support can only be included if a system selects
CONFIG_SYS_HAS_CPU_R8000. No system does, making all R8000-related CPU
support dead code. Remove it.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org