[SpotBugs] Remove unread protected field in Checker

[kaijchen]

What changes were proposed in this pull request?

1. Remove unread protected field in Checker.
2. Remove SpotBugs exclusion URF_UNREAD_PUBLIC_OR_PROTECTED_FIELD.

Why are the changes needed?

Reduce SpotBugs exceptions.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

CI.

[codecov-commenter]

Codecov Report

Merging #520 (85d451d) into master (b80972c) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #520      +/-   ##
============================================
- Coverage     59.76%   59.76%   -0.01%     
  Complexity     1768     1768              
============================================
  Files           205      205              
  Lines         11531    11530       -1     
  Branches       1033     1033              
============================================
- Hits           6892     6891       -1     
  Misses         4233     4233              
  Partials        406      406              

Impacted Files	Coverage Δ
...c/main/java/org/apache/uniffle/server/Checker.java	100.00% <ø> (ø)

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[advancedxy]

Thanks @kaijchen

[kaijchen]

Ref #532