Doc: remove claim that hash index creation depends on effective_cache_size.

Doc: remove claim that hash index creation depends on effective_cache_size.

This text was added by commit ff213239c, and not long thereafter obsoleted
by commit 4adc2f72a (which made the test depend on NBuffers instead); but
nobody noticed the need for an update.  Commit 9563d5b5e adds some further
dependency on maintenance_work_mem, but the existing verbiage seems to
cover that with about as much precision as we really want here.  Let's
just take it all out rather than leaving ourselves open to more errors of
omission in future.  (That solution makes this change back-patchable, too.)

Noted by Peter Geoghegan.

Discussion: <CAM3SWZRVANbj9GA9j40fAwheQCZQtSwqTN1GBTVwRrRbmSf7cg@mail.gmail.com>