chore: Remove join_keys attribute

[felixwang9817]

Signed-off-by: Felix Wang wangfelix98@gmail.com

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

[achals]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

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[codecov-commenter]

Codecov Report

Base: 67.11% // Head: 76.13% // Increases project coverage by +9.01% 🎉

Coverage data is based on head (13c4ad5) compared to base (5ddb83b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3159      +/-   ##
==========================================
+ Coverage   67.11%   76.13%   +9.01%     
==========================================
  Files         175      211      +36     
  Lines       15907    17854    +1947     
==========================================
+ Hits        10676    13593    +2917     
+ Misses       5231     4261     -970     

Flag	Coverage Δ
integrationtests	66.93% <100.00%> (-0.18%)	⬇️
unittests	58.28% <100.00%> (?)

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

Impacted Files	Coverage Δ
sdk/python/feast/entity.py	91.04% <ø> (+6.75%)	⬆️
sdk/python/feast/feature_view.py	96.66% <100.00%> (ø)
sdk/python/feast/inference.py	85.22% <100.00%> (+0.95%)	⬆️
...on/feast/infra/materialization/snowflake_engine.py	92.13% <0.00%> (-0.46%)	⬇️
...offline_stores/contrib/spark_repo_configuration.py	100.00% <0.00%> (ø)
sdk/python/feast/loaders/yaml.py	18.18% <0.00%> (ø)
...s/contrib/mssql_offline_store/tests/data_source.py	46.93% <0.00%> (ø)
...hon/feast/infra/utils/postgres/connection_utils.py	48.00% <0.00%> (ø)
.../feature_repos/universal/online_store/cassandra.py	42.85% <0.00%> (ø)
...thon/feast/infra/online_stores/contrib/postgres.py	32.69% <0.00%> (ø)
... and 102 more

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