Remove deprecated customReports detekt task property - #2811

[arturbosch]

Changed to WIP:

• needs to run with Gradle 5.0
• other Gradle PR must be merged first

[codecov]

Codecov Report

Merging #2944 into master will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #2944      +/-   ##
============================================
- Coverage     79.78%   79.73%   -0.06%     
  Complexity     2506     2506              
============================================
  Files           427      427              
  Lines          7534     7539       +5     
  Branches       1420     1421       +1     
============================================
  Hits           6011     6011              
- Misses          767      772       +5     
  Partials        756      756              

Impacted Files	Coverage Δ	Complexity Δ
.../main/kotlin/io/gitlab/arturbosch/detekt/Detekt.kt	27.90% <0.00%> (-0.67%)	16.00 <0.00> (ø)
...kotlin/io/gitlab/arturbosch/detekt/DetektPlugin.kt	53.84% <0.00%> (-1.84%)	10.00 <0.00> (ø)

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