Remove code which warns about invalid longer chain and enters safe mode
This warning was introduced in b8585384 , but no clear
reason was given why it should be necessary to care much
about invalid chains, let alone degrade our own service
based on their presence.
One possible purpose is to detect hard forks, but going
into safe mode based on a hardcoded limit like an estimated
6 blocks more work seems arbitrary and poorly designed.
Also, this warning code is not covered by tests, and is
causing actual issues for users (as reported in Issue 59
on ABC github). It seems leaving it in could serve as a
possible DoS attack vector more than anything useful.