chore: Remove old way of flagging features as experimental

[adchia]

What this PR does / why we need it:
For a while now, we've been using warnings as the default way to tell users a feature is experimental. This formally deprecates needing feature_store.yaml flags to enable ODFV and lambda feature servers.

Which issue(s) this PR fixes:

Fixes #

[codecov-commenter]

Codecov Report

Merging #3005 (3dfcd43) into master (16957ac) will decrease coverage by 17.89%.
The diff coverage is 54.54%.

@@             Coverage Diff             @@
##           master    #3005       +/-   ##
===========================================
- Coverage   77.83%   59.94%   -17.90%     
===========================================
  Files         194      193        -1     
  Lines       16567    16474       -93     
===========================================
- Hits        12895     9875     -3020     
- Misses       3672     6599     +2927     

Flag	Coverage Δ
integrationtests	?
unittests	59.94% <54.54%> (+0.10%)	⬆️

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

Impacted Files	Coverage Δ
sdk/python/feast/errors.py	68.23% <ø> (-1.71%)	⬇️
sdk/python/feast/feature_store.py	71.26% <50.00%> (-14.46%)	⬇️
sdk/python/feast/flags_helper.py	66.66% <50.00%> (-15.69%)	⬇️
sdk/python/feast/infra/aws.py	23.07% <50.00%> (-16.93%)	⬇️
sdk/python/feast/repo_config.py	77.22% <50.00%> (-10.28%)	⬇️
sdk/python/feast/cli.py	41.69% <100.00%> (+2.08%)	⬆️
...sts/integration/registration/test_universal_cli.py	20.20% <0.00%> (-79.80%)	⬇️
...ts/integration/offline_store/test_offline_write.py	26.08% <0.00%> (-73.92%)	⬇️
...fline_store/test_universal_historical_retrieval.py	28.94% <0.00%> (-71.06%)	⬇️
...ests/integration/e2e/test_python_feature_server.py	29.50% <0.00%> (-70.50%)	⬇️
... and 75 more

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[felixwang9817]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adchia, felixwang9817

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