Fix branding after migrating to MPS 2020.3

[eugenschindler]

In 2020.3, the "idea branding" part and the dependent "jar branding.jar" part in the layout of rcp.branding buildscript (in rcp.build buildsolution) broke down. This needs to be fixed.

[eugenschindler]

@arimer you may also have an idea on how to fix this part. I have just commented out that part so it doesn´t give build errors in the build solution, but it should probably be moved or specified elsewhere in the build solution...