Remove outdated code of task dim bounds

Freeform task is no longer multiple level task. So the condition
inFreeformWindowingMode() behind isRootTask() in getDimBounds
never satisfied. Also the task bounds no longer excludes insets,
so the calculation getMaxVisibleBounds is unnecessary.

Minimized home in split is also removed several years ago. The bounds
of home task and its root task are always the same.

Bug: 163976519
Flag: EXEMPT remove dead code
Test: CtsWindowManagerDeviceActivity
Change-Id: Iee12d7e8be095fbc1149f8ab439896c8900a7b18