alter: zap space_vtab::commit_alter

space_vtab::commit_alter is implemented only by memtx, which uses it
to set bsize for a new space. However, it isn't necessary to use
commit_alter for this - instead we can set bsize in prepare_alter and
reset it in drop_primary_key, along with memtx_space::replace. Let's
do it and zap space_vtab::commit_alter altogether, because the callback
is confusing: judging by its name it should be called after WAL write,
but it is called before.