Remove overzealous VACUUM failsafe assertions.
The failsafe can trigger when index processing is already disabled.
This can happen when VACUUM's INDEX_CLEANUP parameter is "off" and the
failsafe happens to trigger. Remove assertions that assume that index
processing is directly tied to the failsafe.
Oversight in commit
c242baa4, which made it possible for the failsafe to
trigger in a two-pass strategy VACUUM that has yet to make its first
call to lazy_vacuum_all_indexes().