Don't flag inherited visibility in NestedClassesVisibility

[schalkms]

The rule had an issue that it forced the use of an additional
unnecessary modifier. However, the visibility of nested classes is
inherited from the parent class.
These nested classes can't have a higher visibility than their parent.
The following example outlines the issue.

internal class A1 { // non-compliant
class B
}
internal class A2 { // compliant
internal class B
}

Closes #1930

[codecov-io]

Codecov Report

Merging #2261 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #2261      +/-   ##
============================================
- Coverage     81.45%   81.42%   -0.03%     
+ Complexity     2114     2113       -1     
============================================
  Files           349      349              
  Lines          6054     6051       -3     
  Branches       1106     1106              
============================================
- Hits           4931     4927       -4     
  Misses          532      532              
- Partials        591      592       +1

Impacted Files	Coverage Δ	Complexity Δ
...osch/detekt/rules/style/NestedClassesVisibility.kt	83.33% <100%> (-10%)	14 <0> (-1)

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[BraisGabin]

If we want that this rule detects the misleading modifiers we should check the private classes with public or internal classes too.

[schalkms]

@BraisGabin Agreed. Please open a new issue for that. This should be done in a separate PR though.