Mark commented out code as unused

Mark commented out code as unused

There were many PG_GETARG_* calls, mostly around gin, gist, spgist
code, that were commented out, presumably to indicate that the
argument was unused and to indicate that it wasn't forgotten or
miscounted.  But keeping commented-out code updated with refactorings
and style changes is annoying.  So this commit changes them to

    #ifdef NOT_USED

blocks, which is a style already in use.  That way, at least the
indentation and syntax highlighting works correctly, making some of
these blocks much easier to read.

An alternative would be to just delete that code, but there is some
value in making unused arguments explicit, and some of this arguably
serves as example code for index AM APIs.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: David Geier <geidav.pg@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/328e4371-9a4c-4196-9df9-1f23afc900df%40eisentraut.org