The initial implementation of this feature was really unsupportable,
because it's relying on the physical size of an on-disk file to carry the
relation's populated/unpopulated state, which is at least a modularity
violation and could have serious long-term consequences.  We could say that
an unlogged matview goes to empty on crash, but not everybody likes that
definition, so let's just remove the feature for 9.3.  We can add it back
when we have a less klugy implementation.

I left the grammar and tab-completion support for CREATE UNLOGGED
MATERIALIZED VIEW in place, since it's harmless and allows delivering a
more specific error message about the unsupported feature.

I'm committing this separately to ease identification of what should be
reverted when/if we are able to re-enable the feature.