Remove invoke functionality

[mthenw]

What did you implement:

This PR removes some remaining code regarding invoke event handling.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: Yes
Is it a breaking change?: Yes

[codecov]

Codecov Report

Merging #432 into master will increase coverage by 0.59%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #432     +/-   ##
=========================================
+ Coverage   63.89%   64.49%   +0.6%     
=========================================
  Files          30       30             
  Lines        1681     1639     -42     
=========================================
- Hits         1074     1057     -17     
+ Misses        563      538     -25     
  Partials       44       44

Impacted Files	Coverage Δ
event/event.go	77.41% <ø> (ø)	⬆️
internal/cache/subscription_cache.go	86.15% <ø> (-2.88%)	⬇️
router/router.go	55.83% <ø> (+3.68%)	⬆️
internal/cache/target.go	0% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2c5a6b5...01b1a0f. Read the comment docs.

[RaeesBhatti]

LGTM