Removed experimental explain_plan feature.

[antonpirker]

Attaching the database explain plan to a db span was an experimental feature done in an Sentry Hackweek.

As we are moving into an Otel world, we remove this experiment from our Repository.

There is still a branch experiment/explain_plans on Github to keep the code for future reference:
https://github.com/getsentry/sentry-python/tree/experiment/explain_plans

(maybe we can copy the code into the Opentelemetry instrumentation if we want to see this feature in the future)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.45%. Comparing base (e6ca5a2) to head (a48ed4b).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3534      +/-   ##
==========================================
+ Coverage   84.22%   84.45%   +0.23%     
==========================================
  Files         136      133       -3     
  Lines       13900    13833      -67     
  Branches     2940     2927      -13     
==========================================
- Hits        11707    11683      -24     
+ Misses       1465     1422      -43     
  Partials      728      728              

Files with missing lines	Coverage Δ
sentry_sdk/consts.py	99.50% <ø> (ø)
sentry_sdk/integrations/django/__init__.py	85.15% <ø> (+0.38%)	⬆️
sentry_sdk/integrations/sqlalchemy.py	66.66% <ø> (+2.71%)	⬆️

... and 2 files with indirect coverage changes