(Based on original patch by Oleksandr Byelkin)

Multi-table DELETE can execute via "buffered" mode: at phase #1 it collects
rowids of rows to be deleted, then at phase #2 in multi_delete::do_deletes()
it calls handler->rnd_pos() to read rows to be deleted and deletes them.

The problem occurred when phase #1 used Rowid Filter on the table that
phase #2 would be deleting from.
In InnoDB, h->rnd_init(scan=false) and h->rnd_pos() is an index scan over PK
under the hood. So, at phase #2 ha_innobase::rnd_init() would try to use the
Rowid Filter and hit an assertion inside ha_innobase::rnd_init().

Note that multi-table UPDATE works similarly but was not affected, because
patch for MDEV-7487 added code to disable rowid filter for phase #2 in
multi_update::do_updates().

This patch changes the approach:
- It makes InnoDB not use Rowid Filter in rnd_pos() scans: it is disabled in
  ha_innobase::rnd_init() and enabled back in ha_innobase::rnd_end().
- multi_update::do_updates() no longer disables Rowid Filter for phase#2 as
  it is no longer necessary.