gc: do not abort garbage collection if failed to unlink snap file

We build the checkpoint list from the list of memtx snap files. So to
ensure that it is always possible to recover from any checkpoint present
in box.info.gc() output, we abort garbage collection if we fail to
unlink a snap file. This introduces extra complexity to the garbage
collection code, which makes it difficult to make WAL file removal fully
asynchronous.

Actually, it looks like we are being way too overcautious here, because
unlink() doesn't normally fail so an error while removing a snap file is
highly unlikely to occur. Besides, even if it happens, it still won't be
critical, because we never delete the last checkpoint, which is usually
used for backups/recovery. So let's simplify the code by removing that
check.

Needed for #3938