memtx: refactor handling point write

There's a special 'point hole' mechanism in mvcc transactional
manager that manages point gap reads by full key when no raw
tuple was found in the index. For instance, it's the only way
to collect gap reads for non-tree indexes.

Once a new tuple is inserted to the index, the read records are
transferred to the normal read set in the corresponding story.
Actually after that the 'point hole' record in no more needed.

So let's remove it.

While we are here, drop unused point_holes_size, improve names
and comments.

Part of #8648
Part of #8654

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 62c65639)