kdevplatform: remove About data feature

With KDevPlatform no longer a stand-alone product, but effectively
only an internal architecture platform of KDevelop, the About data
and the About KDevPlatform dialog no longer make sense

Any author information is merged with the one of the KDevelop app,
which it already almost was a subset of.
1 job for !84 with removeplatformaboutdata
detached
Status Job ID Name Coverage
  Build
canceled #27458
linux kf5-qt5 qt5.12