largefiles: drop os.path.join() in subrepo archive override

This is related to 41cd8171e58f, but not required for that to function correctly
because 'prefix' always ends with '/', so os.path.join(prefix, _path) simply
concatenates the strings.  Take the opportunity to drop the os.path usage, and
avoid confusion by using the same pattern as the overridden subrepo method.

hgext/largefiles/overrides.py

@@ -985,7 +985,7 b' def hgsubrepoarchive(orig, repo, archive'
     for subpath in sorted(ctx.substate):
         sub = ctx.sub(subpath)
         submatch = match_.narrowmatcher(subpath, match)
-        sub.archive(archiver, os.path.join(prefix, repo._path) + '/', submatch)
+        sub.archive(archiver, prefix + repo._path + '/', submatch)
 
 # If a largefile is modified, the change is not reflected in its
 # standin until a commit. cmdutil.bailifchanged() raises an exception