Remove server side task positioner

The management was moved to shell, as [1] has removed the only caller.
Now the positioner becomes dead code.

[1]: If8ac1cc9e8a81f63f9960768aea8d2016a67c4f9

(The interface of Session will be removed later)

Bug: 163976519
Flag: EXEMPT remove dead code
Test: CtsWindowManagerDeviceActivity
Change-Id: I37752794b64e6a3e56f89c8b9e007d6e7c48cf0b
7 files changed