Remove duplicate package build success message

Previously we would see the following make output:

[ 68%] Built target TrigUpgradeTestClidGen
[ 69%] Built package TrigUpgradeTest
TrigUpgradeTest: Package build succeeded
[ 69%] Built target Package_TrigUpgradeTest

With this change it is simply:

[ 68%] Built target TrigUpgradeTestClidGen
[ 69%] Built package TrigUpgradeTest
[ 69%] Built target Package_TrigUpgradeTest

I think I tried to do this already when I implemented !200 (merged) but the "Built package" line never appeared. I think this must be a bug/feature of the make generator to not ever print something if the COMMAND is empty. So I now simply added a dummy command. I checked that the "Built package" message is indeed printed when the target is done so it appears in the exact same location as the previous echo. The only question is if "Package build succeeded" is somehow a magic string used by NICOS. If yes, we can of course change the "Built package" to something else.