Remove duplicate userAgent string

[bdemers]

Fixes: #135

[bretterer]

LGTM. Thanks for including a test!

[codecov-io]

Codecov Report

Merging #139 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #139      +/-   ##
============================================
- Coverage     60.84%   60.84%   -0.01%     
  Complexity     1312     1312              
============================================
  Files           155      155              
  Lines          5272     5271       -1     
  Branches        836      836              
============================================
- Hits           3208     3207       -1     
  Misses         1719     1719              
  Partials        345      345

Impacted Files	Coverage Δ	Complexity Δ
...java/com/okta/sdk/impl/http/support/UserAgent.java	34.78% <100%> (-0.41%)	22 <0> (ø)

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