Remove deprecated HierarchicalConfig which could lead to OOM when reusing Config objects

[arturbosch]

We have deprecated it three minor versions ago in (1.7.0).

[codecov]

Codecov Report

Merging #2768 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #2768      +/-   ##
============================================
+ Coverage     80.53%   80.56%   +0.02%     
  Complexity     2319     2319              
============================================
  Files           385      385              
  Lines          6961     6955       -6     
  Branches       1261     1261              
============================================
- Hits           5606     5603       -3     
+ Misses          727      724       -3     
  Partials        628      628              

Impacted Files	Coverage Δ	Complexity Δ
...n/kotlin/io/gitlab/arturbosch/detekt/api/Config.kt	100.00% <100.00%> (+5.88%)	0.00 <0.00> (ø)
...itlab/arturbosch/detekt/api/internal/BaseConfig.kt	96.15% <100.00%> (ø)	12.00 <0.00> (ø)
...tlab/arturbosch/detekt/api/internal/EmptyConfig.kt	80.00% <100.00%> (+13.33%)	4.00 <1.00> (ø)
...itlab/arturbosch/detekt/api/internal/YamlConfig.kt	81.48% <100.00%> (+1.48%)	9.00 <0.00> (ø)

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