pgsql: Fix broken dependency-mongering for index operator classes/famil

Fix broken dependency-mongering for index operator classes/families.

For a long time, opclasscmds.c explained that "we do not create a
dependency link to the AM [for an opclass or opfamily], because we don't
currently support DROP ACCESS METHOD". Commit 473b93287040b200 invented
DROP ACCESS METHOD, but it batted only 1 for 2 on adding the dependency
links, and 0 for 2 on updating the comments about the topic.

In passing, undo the same commit's entirely inappropriate decision to
blow away an existing index as a side-effect of create_am.sql.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/92a30a7eb0cadb008e18053f199af7de3fc1abaa

Modified Files
--------------
src/backend/commands/opclasscmds.c | 21 ++++++++----------
src/test/regress/expected/create_am.out | 35 +++++++++++++-----------------
src/test/regress/expected/sanity_check.out | 2 +-
src/test/regress/sql/create_am.sql | 21 ++++++++----------
4 files changed, 34 insertions(+), 45 deletions(-)