feat: allow functions returned from bpd.read_gbq_function to execute outside of apply

[tswast]

Tested manually:

import bigframes.pandas as bpd

func = bpd.read_gbq_function("bqutil.fn.cw_lower_case_ascii_only")
func("AURÉLIE")
# Out[3]: 'aurÉlie'

TODO

• Keep in sync with changes in feat: allow functions decorated with @bpd.remote_function to execute locally #704
• Add tests
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

🦕

[tswast]

Outside of system tests, there weren't any code paths that passed in bigquery_client but not session, so removing this unnecessary logic.