CLN: remove `private_key` and `verbose` from gbq

[charlesdong1991]

Since verbose and private_key have been deprecated since #30200, we should also remove both from function since they are not used at all? see them in read_gbq and to_gbq, e.g.

pandas/pandas/io/gbq.py

Line 33 in 24857a2

private_key=None,

pandas/pandas/io/gbq.py

Line 34 in 24857a2

verbose=None,

[faraz16iqbal]

Is this issue still up for grabs?