fix: remove unnecessary app context on celery

[dpgaspar]

SUMMARY

All celery tasks on superset already have an app context created. https://github.com/apache/superset/blob/master/superset/initialization/__init__.py#L102

Recreating this context will make the task run on a Flask context inside a Flask context.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15422 (e5ddd52) into master (ddcf461) will increase coverage by 0.01%.
The diff coverage is 68.42%.

@@            Coverage Diff             @@
##           master   #15422      +/-   ##
==========================================
+ Coverage   76.99%   77.01%   +0.01%     
==========================================
  Files         975      975              
  Lines       50634    50642       +8     
  Branches     6222     6222              
==========================================
+ Hits        38988    39001      +13     
+ Misses      11435    11430       -5     
  Partials      211      211              

Flag	Coverage Δ
mysql	81.73% <68.42%> (+0.02%)	⬆️
postgres	81.75% <68.42%> (+0.02%)	⬆️
python	81.84% <68.42%> (+0.02%)	⬆️
sqlite	81.37% <68.42%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/tasks/thumbnails.py	45.16% <5.88%> (+2.73%)	⬆️
superset/tasks/async_queries.py	96.77% <95.00%> (-0.25%)	⬇️
superset/app.py	68.42% <0.00%> (-14.92%)	⬇️
superset/config.py	97.03% <0.00%> (+5.88%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ddcf461...e5ddd52. Read the comment docs.