GenericPackage: Remove X-Plasma-MainScript key
Having a Plasma key in KPackage doesn't really make sense due to the Plasma prefix.
Also, this feature has little actual use and when used, it often causes one to guess/have a look at the metadata to find out which file is the entrypoint to the package. For KCMs, this feature was removed when porting them away from KPackage.
For the generic/QML KPackages, this feature seems unused in KF6. I will also remove it in plasma-frameworks, but there porting is required.
Edited by Alexander Lohnau