Remove some unused TimeFieldSpec related methods from Schema

[npawar]

Another step towards: #2756

As I was making changes to Schema to treat TIME as DATE_TIME, found a subset of changes that can go prior to the bigger change.
Removing 3 methods from the Schema which are related to TimeFieldSpec

1. getIncomingTimeUnit - was unused
2. getOutgoingTimeUnit - was unused
3. getTimeColumnName - was able to remove the 4 usages

[mcvsubbu]

ltgm other than minor comment

[codecov-io]

Codecov Report

Merging #5347 into master will decrease coverage by 0.06%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #5347      +/-   ##
==========================================
- Coverage   66.51%   66.45%   -0.07%     
==========================================
  Files        1075     1075              
  Lines       54780    54757      -23     
  Branches     8171     8165       -6     
==========================================
- Hits        36438    36387      -51     
- Misses      15656    15705      +49     
+ Partials     2686     2665      -21     

Impacted Files	Coverage Δ
...ore/minion/rollup/MergeRollupSegmentConverter.java	89.28% <66.66%> (ø)
.../core/indexsegment/mutable/MutableSegmentImpl.java	76.38% <70.00%> (+2.32%)	⬆️
.../columnminmaxvalue/ColumnMinMaxValueGenerator.java	70.00% <100.00%> (-0.43%)	⬇️
...rc/main/java/org/apache/pinot/spi/data/Schema.java	75.17% <100.00%> (+0.08%)	⬆️
...ller/validation/OfflineSegmentIntervalChecker.java	29.48% <0.00%> (-53.85%)	⬇️
...apache/pinot/common/metrics/ValidationMetrics.java	47.45% <0.00%> (-28.82%)	⬇️
.../impl/dictionary/FloatOnHeapMutableDictionary.java	63.41% <0.00%> (-13.42%)	⬇️
...he/pinot/controller/util/SegmentIntervalUtils.java	0.00% <0.00%> (-8.34%)	⬇️
.../org/apache/pinot/core/startree/StarTreeUtils.java	81.08% <0.00%> (-5.41%)	⬇️
...rg/apache/pinot/broker/routing/RoutingManager.java	78.62% <0.00%> (-3.44%)	⬇️
... and 15 more

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