tools: ynl: remove unused imports

tools: ynl: remove unused imports

These imports are not used according to Ruff, and can be safely removed.

This is linked to Ruff error F401 [1]:

  Unused imports add a performance overhead at runtime, and risk
  creating import cycles. They also increase the cognitive load of
  reading the code.

There is one exception with 'YnlDocGenerator' which is added in __all__:
it is used by ynl_gen_rst.py.

Link: https://docs.astral.sh/ruff/rules/unused-import/
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Link: https://patch.msgid.link/20250909-net-next-ynl-ruff-v1-5-238c2bccdd99@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>