MAINT: remove file with note to remove in 0.5.0

[jbrockmendel]

Long overdue

[coveralls]

Coverage increased (+0.01%) to 84.552% when pulling 543261e on jbrockmendel:useless into e1614ff on statsmodels:master.

[codecov]

Codecov Report

Merging #5638 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5638      +/-   ##
==========================================
+ Coverage   82.02%   82.02%   +<.01%     
==========================================
  Files         588      587       -1     
  Lines       93432    93428       -4     
  Branches    10376    10376              
==========================================
  Hits        76635    76635              
+ Misses      14396    14392       -4     
  Partials     2401     2401

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