Revert "Increase the default vacuum_cost_limit from 200 to 2000"

Revert "Increase the default vacuum_cost_limit from 200 to 2000"

This reverts commit bd09503e633b8077822bb4daf91625b71ac16253.

Per discussion, it seems like what we should do instead is to
reduce the default value of autovacuum_vacuum_cost_delay by the
same factor.  That's functionally equivalent as long as the
platform can accurately service the smaller delay request, which
should be true on anything released in the last 10 years or more.
And smaller, more-closely-spaced delays are better in terms of
providing a steady I/O load.

Discussion: https://postgr.es/m/28720.1552101086@sss.pgh.pa.us