Fix memory leak in KBlocksScene

The destructor of KBlocksScene now calls deleteGameItemGroups() to
ensure that the created objects are properly delete. This fixes a
memory leak flagged by valgrind.
1 job for !6 with fix_memory_issues in 4 minutes and 1 second (queued for 91 minutes and 36 seconds)
detached
Status Job ID Name Coverage
  Build
passed #92021
linux kf5-qt5 qt5.15

00:04:01

15.0%