Enable UnnecessaryLet rule for detekt code base

[marschwar]

Yet another one

[codecov]

Codecov Report

Merging #4024 (3d6f92c) into main (48c1da8) will increase coverage by 0.00%.
The diff coverage is 57.14%.

@@            Coverage Diff            @@
##               main    #4024   +/-   ##
=========================================
  Coverage     83.55%   83.55%           
- Complexity     3181     3182    +1     
=========================================
  Files           459      459           
  Lines          9085     9087    +2     
  Branches       1771     1771           
=========================================
+ Hits           7591     7593    +2     
  Misses          561      561           
  Partials        933      933           

Impacted Files	Coverage Δ
...n/io/github/detekt/metrics/CyclomaticComplexity.kt	94.44% <0.00%> (ø)
...rbosch/detekt/rules/complexity/NestedBlockDepth.kt	91.83% <0.00%> (ø)
...tlab/arturbosch/detekt/rules/naming/NamingRules.kt	93.65% <0.00%> (ø)
...lab/arturbosch/detekt/api/internal/Suppressions.kt	74.07% <100.00%> (+0.99%)	⬆️
...ules/documentation/LicenceHeaderLoaderExtension.kt	80.95% <100.00%> (+0.46%)	⬆️

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