Fix false positive for UnnecessaryLet with disabled type resolution

[cortinico]

The fix is the same we applied for #2938 (restricting the rule to type resolution only).

Fixes #2987

[BraisGabin]

Could you add this example to the tests?

val a = listOf<List<String>?>(listOf(""))
    .map { list -> list?.let { it + it } }

To ensure that in the future we just remove the if thinking "it's not necessary"

[codecov]

Codecov Report

Merging #2991 into master will not change coverage.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master    #2991   +/-   ##
=========================================
  Coverage     80.29%   80.29%           
+ Complexity     2478     2477    -1     
=========================================
  Files           424      424           
  Lines          7479     7479           
  Branches       1370     1370           
=========================================
  Hits           6005     6005           
  Misses          760      760           
  Partials        714      714           

Impacted Files	Coverage Δ	Complexity Δ
...ab/arturbosch/detekt/rules/style/UnnecessaryLet.kt	80.43% <0.00%> (-1.79%)	13.00 <0.00> (ø)
...n/kotlin/io/gitlab/arturbosch/detekt/rules/Junk.kt	46.66% <0.00%> (-6.67%)	0.00% <0.00%> (ø%)
...lab/arturbosch/detekt/formatting/FormattingRule.kt	96.42% <0.00%> (+6.77%)	11.00% <0.00%> (-1.00%)	⬆️

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