Remove duplicate error code NON_DROPFRAME_RATE.

[ssteinbach]

Closes #625

RationalTime had a duplicate error code in it that was only used in one
place.

[codecov-commenter]

Codecov Report

Merging #1114 (4201d6b) into master (1dbe0fc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1114   +/-   ##
=======================================
  Coverage   86.21%   86.21%           
=======================================
  Files         191      191           
  Lines       19030    19029    -1     
  Branches     2105     2105           
=======================================
  Hits        16406    16406           
+ Misses       2078     2077    -1     
  Partials      546      546           

Flag	Coverage Δ
unittests	86.21% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/opentime/errorStatus.cpp	55.55% <ø> (+5.55%)	⬆️
src/opentime/errorStatus.h	100.00% <ø> (ø)
tests/test_opentime.py	99.72% <ø> (ø)
src/opentime/rationalTime.cpp	82.75% <100.00%> (ø)

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