Remove obsolete name_ops index-only scan comments.

Remove obsolete name_ops index-only scan comments.

nbtree index-only scans of an index that uses btree/name_ops as one of
its index column's input opclasses are no longer at any risk of reading
past the end of currTuples.  We're no longer reliant on such scans being
able to at least read from the start of markTuples storage (which uses
space from the same allocation as currTuples) to avoid a segfault:
StoreIndexTuple (from nodeIndexonlyscan.c) won't actually read past the
end of a cstring datum from a name_ops index.  In other words, we
already have the "special-case treatment for name_ops" that the removed
comment supposed we could avoid by relying on markTuples in this way.

Oversight in commit a63224be49, which added special case handling of
name_ops cstrings to StoreIndexTuple, but missed these comments.