Remove unused parents property of Entity

[kmax12]

This PR removes the unused parents property of the Entity class.

The was one usage of it, but that was refactored

Also, remove test case

[codecov-io]

Codecov Report

Merging #283 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
+ Coverage   94.44%   94.44%   +<.01%     
==========================================
  Files          71       71              
  Lines        7719     7711       -8     
==========================================
- Hits         7290     7283       -7     
+ Misses        429      428       -1

Impacted Files	Coverage Δ
featuretools/tests/entityset_tests/test_entity.py	100% <ø> (ø)	⬆️
featuretools/entityset/entity.py	93.45% <ø> (-0.04%)	⬇️
featuretools/entityset/entityset.py	93.8% <100%> (ø)	⬆️
featuretools/utils/gen_utils.py	70.58% <0%> (+1.96%)	⬆️

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[rwedge]

Looks good!