Remove old OC_Hook for OC_Group events

Those mappings exist and we will remove the first ones (labeled as `old`):

old: `\OC_Hook::listen('OC_Group', 'pre_createGroup', array('run' => true, 'gid' => $gid));`
since OC 8 (owncloud/core#12618): `$groupManager->listen('\OC\Group', 'preCreate', function ($gid) { ... });`
since NC 17 (#18350): `OCP\Group\Events\BeforeGroupCreatedEvent`

old: `\OC_Hook::emit('OC_User', 'post_createGroup', array('gid' => $gid->getGID()));`
since OC 8 (owncloud/core#12618): `$groupManager->listen('\OC\Group', 'postCreate', function (\OC\Group\Group $gid) { ... });`
since NC 17 (#18350): `OCP\Group\Events\GroupCreatedEvent`

old: `\OC_Hook::emit('OC_Group', 'pre_deleteGroup', array('run' => true, 'gid' => $group->getGID()));`
since OC 8 (owncloud/core#12618): `$groupManager->listen('\OC\Group', 'preDelete', function (\OC\Group\Group $group) { ... });`
since NC 17 (#18350): `OCP\Group\Events\BeforeGroupDeletedEvent`

old: `\OC_Hook::emit('OC_User', 'post_deleteGroup', array('gid' => $group->getGID()));`
since OC 8 (owncloud/core#12618): `$groupManager->listen('\OC\Group', 'postDelete', function (\OC\Group\Group $group) { ... });`
since NC 17 (#18350): `OCP\Group\Events\GroupDeletedEvent`

old: `\OC_Hook::emit('OC_Group', 'pre_addToGroup', array('run' => true, 'uid' => $user->getUID(), 'gid' => $group->getGID()));`
since OC 8 (owncloud/core#12618): `$groupManager->listen('\OC\Group', 'preAddUser', function (\OC\Group\Group $group, \OC\User\User $user) { ... });`
since NC 17 (#18350): `OCP\Group\Events\BeforeUserAddedEvent`

old: `\OC_Hook::emit('OC_Group', 'post_addToGroup', array('uid' => $user->getUID(), 'gid' => $group->getGID()));`
since OC 8 (owncloud/core#12618): `$groupManager->listen('\OC\Group', 'postAddUser', function (\OC\Group\Group $group, \OC\User\User $user) { ... });`
since NC 17 (#18350): `OCP\Group\Events\UserAddedEvent`

Signed-off-by: Morris Jobke <hey@morrisjobke.de>