Fix btree mark/restore bug.

Commit 2ed5b87f introduced a bug in
mark/restore, in an attempt to optimize repeated restores to the
same page.  This caused an assertion failure during a merge join
which fed directly from an index scan, although the impact would
not be limited to that case.  Revert the bad chunk of code from
that commit.

While investigating this bug it was discovered that a particular
"paranoia" set of the mark position field would not prevent bad
behavior; it would just make it harder to diagnose.  Change that
into an assertion, which will draw attention to any future problem
in that area more directly.

Backpatch to 9.5, where the bug was introduced.

Bug #14169 reported by Shinta Koyanagi.
Preliminary analysis by Tom Lane identified which commit caused
the bug.