pgsql: Remove obsolete nbtree relation extension comment.

Remove obsolete nbtree relation extension comment.

Commit 0d1fe9f7 improved the approach that vacuumlazy.c takes when it
encounters an empty heap page.  It no acquires the relation extension
lock.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0f6aa893cb58c2a5a92016914c94865635345a22

Modified Files
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src/backend/access/nbtree/nbtree.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)