Removed unused exception class, fixes #227

[Roach]

Summary

Fixes #227
Removing an unused exception class, which is also misspelled.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #233 into master will decrease coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
- Coverage   59.61%   59.35%   -0.27%     
==========================================
  Files           9        9              
  Lines         312      310       -2     
  Branches       57       57              
==========================================
- Hits          186      184       -2     
  Misses        112      112              
  Partials       14       14

Impacted Files	Coverage Δ
slackclient/server.py	58.4% <ø> (-0.66%)	⬇️

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