fix(eslint-plugin): [no-unused-vars] handle TSCallSignature

[bradzacher]

Fixes #2331

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #2336 into v4 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##               v4    #2336   +/-   ##
=======================================
  Coverage   92.97%   92.97%           
=======================================
  Files         285      285           
  Lines        9014     9014           
  Branches     2510     2510           
=======================================
  Hits         8381     8381           
  Misses        304      304           
  Partials      329      329           

Flag	Coverage Δ
#unittest	92.97% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/no-unused-vars.ts	96.87% <ø> (ø)