remove unnecessary PaX detection

Rust no longer has support for the LLVM JIT compiler, so it doesn't
currently require a PaX MPROTECT exception. The extended attributes are
preferred over modifying the binaries so it's not actually going to work
on most systems like this anyway.

If JIT compilation ends up being supported again, it should handle this
by *always* applying the exception via an extended attribute without
performing auto-detection of PaX on the host. The `paxctl` tool is only
necessary with the older method involving modifying the ELF binary.