Remove redundant code in BigQueryHookAsync

[phanikumv]

This PR addresses the code changes needed to use the OSS BigQueryHook within the repo instead of using a vendored hook

• Remove _BigQueryHook usage
• Change the tests accordingly for the google async hooks and operators

Closes: #52

[codecov]

Codecov Report

Merging #624 (864897d) into main (a96d071) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #624      +/-   ##
==========================================
+ Coverage   98.24%   98.26%   +0.01%     
==========================================
  Files          73       73              
  Lines        3994     3969      -25     
==========================================
- Hits         3924     3900      -24     
+ Misses         70       69       -1     

Impacted Files	Coverage Δ
...omer/providers/google/cloud/extractors/bigquery.py	100.00% <100.00%> (ø)
...stronomer/providers/google/cloud/hooks/bigquery.py	100.00% <100.00%> (+0.79%)	⬆️
...nomer/providers/google/cloud/operators/bigquery.py	100.00% <100.00%> (ø)

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[pankajkoti]

Nice cleanup 👍🏽