Delete stored fields from index stats

[PepijnBoers]

Deletes stored fields from index statistics, so that we only return quantitative stats.

[codecov]

Codecov Report

Merging #1219 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1219      +/-   ##
============================================
+ Coverage     48.18%   48.35%   +0.16%     
- Complexity      732      739       +7     
============================================
  Files           147      147              
  Lines          8552     8547       -5     
  Branches       1213     1212       -1     
============================================
+ Hits           4121     4133      +12     
+ Misses         4093     4074      -19     
- Partials        338      340       +2     

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/anserini/index/IndexReaderUtils.java	52.23% <ø> (-1.05%)	30.00 <0.00> (-1.00)
...anserini/ltr/feature/base/PMIFeatureExtractor.java	84.61% <0.00%> (-1.93%)	12.00% <0.00%> (-1.00%)
...java/io/anserini/ltr/feature/CountBigramPairs.java	89.61% <0.00%> (+23.37%)	33.00% <0.00%> (+9.00%)

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