workingctx: eliminate remove function
Inlining it into it's last remaining call place in cmdutil.copy.
Note that cmdutil.copy is called with the wlock already held, so no additional
locking is needed to call util.unlinkpath.
We do not need to wrap the util.unlinkpath call into a try block, because
at that point we already know whether abssrc exists or not -- thanks to the
preceding util.copyfile call. Adding a new local 'srcexists' in cmdutil.copy
for that purpose.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.