pgsql: Remove pg_am.amindexnulls.

Remove pg_am.amindexnulls.

The only use we have had for amindexnulls is in determining whether an
index is safe to cluster on; but since the addition of the amclusterable
flag, that usage is pretty redundant.

In passing, clean up assorted sloppiness from the last patch that touched
pg_am.h: Natts_pg_am was wrong, and ambuildempty was not documented.

Branch
------
master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=7e2f906201c8bb95f7fb17e56b8740c38bda5441

Modified Files
--------------
doc/src/sgml/catalogs.sgml | 14 +++++-----
doc/src/sgml/indexam.sgml | 12 ++++-----
src/backend/commands/cluster.c | 37 ------------------------------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_am.h | 46 ++++++++++++++++++-------------------
5 files changed, 35 insertions(+), 76 deletions(-)