Removed unused methods from TweetNaclFast

[daniellevass]

Description of the pull request

When we added the TweetNaclFast dependency, we added a lot of things we're not using (we're just decrypting, we're not encrypting any messages) - I've removed all the things I don't think we're using.
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Why is the change necessary?

It reduces the amount of code other people need to install from us - it also improves our code coverage metric.
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[codecov-commenter]

Codecov Report

Merging #275 into master will increase coverage by 30.82%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             master     #275       +/-   ##
=============================================
+ Coverage     45.43%   76.25%   +30.82%     
  Complexity      284      284               
=============================================
  Files            31       31               
  Lines          3053     1798     -1255     
  Branches        254      137      -117     
=============================================
- Hits           1387     1371       -16     
+ Misses         1610      371     -1239     
  Partials         56       56               

Impacted Files	Coverage Δ	Complexity Δ
...a/com/pusher/client/crypto/nacl/TweetNaclFast.java	93.61% <ø> (+60.10%)	26.00 <0.00> (ø)

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