Remove unused java12 sourceSet configuration from server (#59151)
Remove unused java12 sourceSet configuration from server (#59151)

The sources have been removed before. This just cleans up the build accordingly by removing the sourceSet and gradle tasks that come along with it 
nknize/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch