btrfs: remove unused btrfs_folio_start_writer_lock()

This function is not really suitable to lock a folio, as it lacks the
proper mapping checks, thus the locked folio may not even belong to
btrfs.

And due to the above reason, the last user inside lock_delalloc_folios()
is already removed, and we can remove this function.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>