A more MPI-IO friendly remove file

MPI-IO allows prefxing file names with something-something: (e.g. pvfs2:
or bglockless:). These prefixed file names, however, do not make sense
to posix calls like access().  So, if we must use access(), strip the
prefix first.  Amended to handle the no-prefix case as well.  Reported
by Florin Isaila <florin.isaila@gmail.com>, who sent me a first draft
of this patch.