pgsql: Remove unnecessary calls of FlushRelationBuffers: there is no

Log Message:
-----------
Remove unnecessary calls of FlushRelationBuffers: there is no need
to write out data that we are about to tell the filesystem to drop.
smgr_internal_unlink already had a DropRelFileNodeBuffers call to
get rid of dead buffers without a write after it's no longer possible
to roll back the deleting transaction. Adding a similar call in
smgrtruncate simplifies callers and makes the overall division of
labor clearer. This patch removes the former behavior that VACUUM
would write all dirty buffers of a relation unconditionally.

Modified Files:
--------------
pgsql/src/backend/access/nbtree:
nbtree.c (r1.124 -> r1.125)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtree.c.diff?r1=1.124&r2=1.125)
pgsql/src/backend/catalog:
heap.c (r1.280 -> r1.281)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c.diff?r1=1.280&r2=1.281)
index.c (r1.247 -> r1.248)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c.diff?r1=1.247&r2=1.248)
pgsql/src/backend/commands:
cluster.c (r1.132 -> r1.133)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/cluster.c.diff?r1=1.132&r2=1.133)
tablecmds.c (r1.147 -> r1.148)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.147&r2=1.148)
vacuum.c (r1.304 -> r1.305)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.304&r2=1.305)
vacuumlazy.c (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c.diff?r1=1.50&r2=1.51)
pgsql/src/backend/storage/buffer:
bufmgr.c (r1.187 -> r1.188)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.187&r2=1.188)
pgsql/src/backend/storage/smgr:
smgr.c (r1.85 -> r1.86)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/smgr/smgr.c.diff?r1=1.85&r2=1.86)
pgsql/src/include/storage:
bufmgr.h (r1.92 -> r1.93)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufmgr.h.diff?r1=1.92&r2=1.93)