Remove silent ignore for skip_locked w/ unsupported backends

Remove silent ignore for skip_locked w/ unsupported backends

For SQLAlchemy 1.4:

The "skip_locked" keyword used with ``with_for_update()`` will render "SKIP
LOCKED" on all MySQL backends, meaning it will fail for MySQL less than
version 8 and on current MariaDB backends.  This is because those backends
do not support "SKIP LOCKED" or any equivalent, so this error should not be
silently ignored.   This is upgraded from a warning in the 1.3 series.

For SQLAlchemy 1.3:

The "skip_locked" keyword used with ``with_for_update()`` will emit a
warning when used on MariaDB backends, and will then be ignored.   This is
a deprecated behavior that will raise in SQLAlchemy 1.4, as an application
that requests "skip locked" is looking for a non-blocking operation which
is not available on those backends.

Fixes: #5578
Change-Id: I49ccb6c6ff46eafed12b77f51e1da8e0e397966c