Remove /twitchemotes/ endpoints

[pajlada]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

• Remove /twitchemotes/ endpoints
• Remove the unused twitchusername cache
• Add changelog entry

Fixes #332

[pajlada]

@M4tthewDE Could you review please?

[codecov]

Codecov Report

Merging #465 (1f20275) into master (4fe8a81) will increase coverage by 1.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
+ Coverage   45.45%   46.70%   +1.25%     
==========================================
  Files         103       99       -4     
  Lines        3819     3691     -128     
==========================================
- Hits         1736     1724      -12     
+ Misses       2033     1918     -115     
+ Partials       50       49       -1     

Impacted Files	Coverage Δ
cmd/api/main.go	0.00% <ø> (ø)
pkg/config/config.go	41.05% <ø> (-0.62%)	⬇️

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[M4tthewDE]

I believe the config option twitchemotes-emote-cache-duration can be removed as well.
Looks good to me other than that. 👍