Clean up AStyleFormatter declarations

* remove unused data member m_indentString;
* remove two unused member functions;
* make two member functions const;
* protected -> private as there are no derived classes.
1 job for !116 with clean-up-formatter-code in 60 minutes (queued for 1 second)
Status Job ID Name Coverage
failed #39061
linux kf5-qt5 qt5.12


Name Stage Failure
linux kf5-qt5 qt5.12 Build The script exceeded the maximum execution time set for the job
kf5.kio.trash: calling moveToTrash( "/tmp/kdevGit_testdir/dir/bar"   0   "bar" )
kdeinit5: PID 25971 terminated.
kf5.kinit.klauncher: 25971 exitStatus= 0
kf5.kinit.klauncher: found no pending requests for PID 25971
113/123 Test #113: test_kdevgit ............................ Passed 23.90 sec
Start 114: test_kdevperforce
kf5.kinit.klauncher: new app ":1.70"
Pulling docker image gitlab/gitlab-runner-helper:x86_64-1b659122 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds