pgsql: Drop the vestigial "smgr" type.

Drop the vestigial "smgr" type.

Before commit 3fa2bb31 this type appeared in the catalogs to
select which of several block storage mechanisms each relation
used.

New features under development propose to revive the concept of
different block storage managers for new kinds of data accessed
via bufmgr.c, but don't need to put references to them in the
catalogs. So, avoid useless maintenance work on this type by
dropping it. Update some regression tests that were referencing
it where any type would do.

Discussion: https://postgr.es/m/CA%2BhUKG%2BDE0mmiBZMtZyvwWtgv1sZCniSVhXYsXkvJ_Wo%2B83vvw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/91595f9d49cf6fd6b6524f3269b2105b3ec18d96

Modified Files
--------------
src/backend/storage/smgr/Makefile | 2 +-
src/backend/storage/smgr/README | 6 ---
src/backend/storage/smgr/smgrtype.c | 80 -------------------------------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 13 -----
src/include/catalog/pg_type.dat | 7 ---
src/test/regress/expected/alter_table.out | 20 ++++----
src/test/regress/expected/type_sanity.out | 3 +-
src/test/regress/sql/alter_table.sql | 8 ++--
9 files changed, 17 insertions(+), 124 deletions(-)