pgsql: Remove legacy multixact truncation support.

Remove legacy multixact truncation support.

In 9.5 and master there is no need to support legacy truncation. This is
just committed separately to make it easier to backpatch the WAL logged
multixact truncation to 9.3 and 9.4 if we later decide to do so.

I bumped master's magic from 0xD086 to 0xD088 and 9.5's from 0xD085 to
0xD087 to avoid 9.5 reusing a value that has been in use on master while
keeping the numbers increasing between major versions.

Discussion: 20150621192409(dot)GA4797(at)alap3(dot)anarazel(dot)de
Backpatch: 9.5

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/aa29c1ccd9f785f9365809f5133e5491acc7ae53

Modified Files
--------------
src/backend/access/transam/multixact.c | 77 ++++++--------------------------
src/backend/access/transam/xlog.c | 4 --
src/backend/commands/vacuum.c | 2 +-
src/include/access/multixact.h | 2 +-
src/include/access/xlog_internal.h | 2 +-
5 files changed, 17 insertions(+), 70 deletions(-)