Partially revert "seccomp: add mmap and address family restrictions for MIPS" (#8563)

This reverts the mmap parts of f5aeac14,
but keeps the part which restricts address families which works
correctly.

Unfortunately the MIPS toolchains still do not implement PT_GNU_STACK.
This means that while the commit to restrict mmap on MIPS was "correct",
it had the side effect of causing pthread_create to fail because glibc tries
to allocate an executable stack for new threads in the absense of
PT_GNU_STACK. We should wait until PT_GNU_STACK is implemented in all
the relevant parts of the toolchain (at least gcc and glibc) before
enabling this again.