x86: Remove unnecessary <stap-probe.h> include from lowlevellock.h

In the i386 case, it appears that the sole remaining LIBC_PROBE was
removed in commit a9fe4c5a ("Support
six-argument syscalls from C for 32-bit x86, use generic
lowlevellock-futex.h (bug 18138)."), when
sysdeps/unix/sysv/linux/i386/lowlevellock-futex.h was replaced with
the generic version.

For x86_64, the relevant change is commit
76f71081 ("Use generic
lowlevellock-futex.h in x86_64 lowlevellock.h."), again by using the
generic version of <lowlevellock-futex.h>.

Tested on i386 and x86_64, with and without --enable-systemtap.