Undo temporary changes for psr/log ^2 support

[Seldaek]

#1587 did some hackery:

• removed rollbar/rollbar from require-dev, this should be rolled back when Allow psr/log 2 rollbar/rollbar-php#536 is merged, and rollbar should be removed from phpstan.neon
• inlined psr/log's LoggerInterfaceTest, when Port Tests php-fig/log-util#2 is merged, the PsrLogCompatTest should be made to use it again.

See also #1589