Fix deleting function that is used as authorizer

[mthenw]

What did you implement:

Closes #473

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?: NO

[codecov]

Codecov Report

Merging #474 into master will decrease coverage by 0.04%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
- Coverage   70.71%   70.66%   -0.05%     
==========================================
  Files          35       35              
  Lines        2165     2175      +10     
==========================================
+ Hits         1531     1537       +6     
- Misses        571      574       +3     
- Partials       63       64       +1

Impacted Files	Coverage Δ
function/errors.go	0% <0%> (ø)	⬆️
httpapi/httpapi.go	90.52% <100%> (+0.04%)	⬆️
libkv/function.go	83.9% <66.66%> (-1.28%)	⬇️

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