Revert "Generate correct RRD filenames for nested graphs. Fixes #1224"

This reverts commit 27f90ac65d388a00d1beabdc5423e2095ad8a2c9.

The fix introduced a regression on subgroups, reverting it. A proper fix is
still to be made.
Merge branch 'pull/162' into stable-2.0
2.0.21