Don't warn on multi delete rowcount if supports_sane_multi is False

Don't warn on multi delete rowcount if supports_sane_multi is False

Fixed an issue where the "number of rows matched" warning would emit even if
the dialect reported "supports_sane_multi_rowcount=False", as is the case
for psycogp2 with ``use_batch_mode=True`` and others.

Fixes: #4661
Change-Id: I93aaf7f597b6083e860ab3cbcd620ba5621c57a8
(cherry picked from commit aa94afcdaf4faeacf13836de2475954e06a7fb67)