Clean up Pandas Backend

[kmax12]

This PR removes some unused lines from Pandas Backend.

Just a minor change to clean up code. No API changes

[codecov-io]

Codecov Report

Merging #302 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
+ Coverage   95.13%   95.21%   +0.08%     
==========================================
  Files          71       71              
  Lines        7702     7687      -15     
==========================================
- Hits         7327     7319       -8     
+ Misses        375      368       -7

Impacted Files	Coverage Δ
...turetools/computational_backends/pandas_backend.py	96.89% <ø> (+2.76%)	⬆️
featuretools/utils/gen_utils.py	85.71% <0%> (-2.39%)	⬇️

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[rwedge]

Looks good