pgsql: Fix nbtree page deletion error messages.

Fix nbtree page deletion error messages.

Adjust some "can't happen" error messages that assumed that the page
deletion target page must be a half-dead page. This assumption was
wrong in the case of an internal target page. Simply refer to these
pages as the target page instead.

Internal pages are never marked half-dead. There is exactly one
half-dead page for each subtree undergoing deletion. The half-dead page
is also the target subtree's leaf-level page. This has been the case
since commit efada2b8, which totally overhauled nbtree page deletion.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3d8d5787a358156edaa7782f0c88e231af974a01

Modified Files
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src/backend/access/nbtree/nbtpage.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)