DOC: remove gbq_integration instructions from contributing.rst
DOC: remove vbench instructions from contributing.rst

Might be useful to include some info on the gbq ci setup? So devs at least have instruction on how to run locally - since these tests only run on master (they are skipped on PR branches). wdyt? 

(I'm happy to add if needed)

Ref:
https://github.com/pandas-dev/pandas/pull/30447