memtx: refactor `memtx_tx_unlink_top_on_space_delete`

The helper consists of `unlink_top_common` and
`unlink_top_on_space_delete_light` helpers. It would make sense if
`unlink_top_common` would be used in any other place, but it's used only
here actually. Let's inline this helpers - it will be easier to read
this code and it will allow to simply patch this function in future
commits.

NO_TEST=refactoring
NO_CHANGELOG=refactoring
NO_DOC=refactoring

(cherry picked from commit 7b2243828937413db9b1c3ffcf8abd8fd92daf02)