Add routine to free MCVList

Add routine to free MCVList

This addition is in the same spirit as 32e27bd32082 for MVNDistinct and
MVDependencies, except that we were missing a free routine for the third
type of extended statistics, MCVList.  I was not sure if we needed an
equivalent for MCVList, but after more review of the main patch set for
the import of extended statistics, it has become clear that we do.

This is introduced as its own change as this routine can be useful on
its own.  This one is a piece that has not been written by Corey
Huinker, I have just noticed it by myself on the way.

Author: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CADkLM=dpz3KFnqP-dgJ-zvRvtjsa8UZv8wDAQdqho=qN3kX0Zg@mail.gmail.com