MIPS: r4k-bugs64: Drop CONFIG_CPU_MIPSR6 checks

The r4k-bugs64 code will no longer be built for MIPSr6 kernel
configurations, so there's no need to perform checks for MIPSr6 within
the code. Drop those redundant checks.

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