Simplify tab completion of extension versions.

Simplify tab completion of extension versions.

Second thoughts about 9cd43f6cb: given that we're staying bug-compatible
with the old behavior of using double not single quotes for extension
versions, we can simplify this completion code by pretending that
extension versions *are* identifiers, and not using VERBATIM.  Then
_complete_from_query() will think that the query results are identifiers
in need of quoting, and we end up with the same behavior as before.

This doesn't work for Query_for_list_of_available_extension_versions_with_TO,
but let's just drop that: there is no other place where we handle
multi-keyword phrases that way, and it doesn't seem very desirable here
either.  Handle completion of "UPDATE TO" in our more usual pattern.

Discussion: https://postgr.es/m/CAMkU=1yV+egSYrzWvbDY8VZ6bKEMrKbzxr-HTuiHi+wDgSUMgA@mail.gmail.com