alpha: Remove nptl/fork.c

The merge at ab21431318d99c94e644606dee1e6a4545d98007 failed
to properly remove the file.
alpha: Merge standard and nptl clone.S
alpha: Consolidate NPTL/non versions of vfork
alpha: Move remaining files out of sysdeps/unix/sysv/linux/alpha/nptl/