Remove deprecated disable_validation property from Trainer

[kaushikb11]

What does this PR do?

Part of #10312

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[tchaton]

LGTM ! But I think this properly was quite useful though.

[codecov]

Codecov Report

Merging #10450 (f96bff6) into master (8b0cb47) will decrease coverage by 0%.
The diff coverage is n/a.

❗ Current head f96bff6 differs from pull request most recent head 6f4244f. Consider uploading reports for the commit 6f4244f to get more accurate results

@@           Coverage Diff           @@
##           master   #10450   +/-   ##
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- Coverage      93%      92%   -0%     
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  Files         178      179    +1     
  Lines       16196    16284   +88     
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+ Hits        14988    15056   +68     
- Misses       1208     1228   +20