fix DroxPerms interaction with primary group

This adds an option that tries to guess if the Main-group of a Player
should be changed. This feature is disabled by default to not break
backward-compatibility.
To activate this, the following needs to be added to the DroxPerms
config file:

Vault:
  useOnlySubgroups: false

remove unused DroxPerms jar
Merge pull request #443 from hydrox/fixDroxPermsPrimaryGroupProblem

Fix DroxPerms interaction with primary group