cleanup unused imports

[trustsafe]

This removes several unused imports.

[lelit]

All those files are generated by tools/extract_enum.py, each one distilled by a different C header in the PostgreSQL source. From where I stand, I see the as a single subpackage (in the sense of from pglast.enum import Foo), splitted into several sources just for convenience.

So, unless you are going to over-complicate (IMHO) the logic used by that script to emit the imports at the beginning of the generated file taking into account whether or not a particular enum subclass is used or not, manually curing each single file with no evident advantage is not going to fly.

If you have particular reasons to avoid the useless imports, I would suggest instead changing extract_enum.py so that it simply emits a single import enum at the top, and the rest of the definitions use the fully qualified enum.Foo.

Thank you.