test: remove replica-applier-rollback.lua

The script name was too long. It was also used as a name for the
unix socket file on which the replica listens. As a result, the
test couldn't start, at least on my machine.

Besides, the script was not any different from the existing
replica.lua, except a couple of not important settings.

The patch drops it and makes gh-4730-applier-rollback.test.lua use
replica.lua. Now it can run on my machine.

Done as a preparation for #6027, which is slightly related to the
test - it is also about errors in applier and their display.