remove.c: combine two helper functions, to avoid sign-extension

remove.c: combine two helper functions, to avoid sign-extension

* src/remove.c (fs_handles_readdir_ordered_dirents_efficiently):
Remove function, so as not to have to worry about the type of
statfs.f_type and sign extension.
(dirent_inode_sort_may_be_useful): Adjust comment.
Perform the switch directly on the struct.member here, instead.
Andreas Schwab spotted the potential for a sign-extension bug,
that happens not to bite for the S_* f_type values currently used.