pgsql: Code cleanup for heap_freeze_tuple.

Code cleanup for heap_freeze_tuple.

It used to be case that lazy vacuum could call this function with only
a shared lock on the buffer, but neither lazy vacuum nor any other
code path does that any more. Simplify the code accordingly and clean
up some related, obsolete comments.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7386089d23c748af142ec7e3668fa0dd164eaf99

Modified Files
--------------
src/backend/access/heap/heapam.c | 49 +++------------------------------
src/backend/access/heap/rewriteheap.c | 7 +----
src/backend/commands/vacuumlazy.c | 3 +-
src/include/access/heapam.h | 3 +-
4 files changed, 7 insertions(+), 55 deletions(-)