fix: remove unnecessary global variable definition

[Linchin]

A new rule in flake8 (F824) has caused our lint test to fail with the following error message, because we defined a global variable in some methods, but we didn't assign any value to them. In this case, you can access the global variable directly so it's unnecessary to define it.

nox > flake8 pandas_gbq tests
pandas_gbq/gbq.py:274:9: F824 `global context` is unused: name is never assigned in scope
pandas_gbq/gbq.py:872:5: F824 `global context` is unused: name is never assigned in scope
nox > Command flake8 pandas_gbq tests failed with exit code 1

Example log: https://github.com/googleapis/python-bigquery-pandas/actions/runs/14229816667/job/39877861461?pr=899

Note: We do change values of some fields to global context, but we do so by accessing its property, and context was not reassigned, so F824 still applies here.

[sycai]

Thanks!

[chalmerlowe]

LGTM