Remove OrderingFilter.help_text

[rpkilby]

Fixes #756.

[codecov-io]

Codecov Report

Merging #757 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #757      +/-   ##
===========================================
+ Coverage    98.13%   98.13%   +<.01%     
===========================================
  Files           15       15              
  Lines         1125     1126       +1     
===========================================
+ Hits          1104     1105       +1     
  Misses          21       21

Impacted Files	Coverage Δ
django_filters/filters.py	98.48% <100%> (ø)	⬆️

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