UnnecessaryLet: report when implicit parameter isn't used

[t-kameyama]

Fixes #3786

[codecov]

Codecov Report

Merging #3794 (77fc8fb) into main (2fbac45) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #3794   +/-   ##
=========================================
  Coverage     83.51%   83.51%           
  Complexity     2914     2914           
=========================================
  Files           452      452           
  Lines          8758     8760    +2     
  Branches       1665     1665           
=========================================
+ Hits           7314     7316    +2     
  Misses          542      542           
  Partials        902      902           

Impacted Files	Coverage Δ	Complexity Δ
...ab/arturbosch/detekt/rules/style/UnnecessaryLet.kt	82.35% <100.00%> (+0.72%)	13.00 <0.00> (ø)

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