Unlink the tombstone proto file before linking the temporary fd.

We were already doing this for the text tombstones but not for protos,
which meant that we stopped producing protos once we hit the limit
on the number of tombstones. Move the code for the text tombstones
into a common location and call it for both types.

Change-Id: I4951150da51a32d50821d147458fc5c18200c9d4