sched: hmp: Remove the global sysctl_sched_enable_colocation tunable

Colocation in HMP includes a tunable that turns on or off the feature
globally across all colocation groups. Supporting this tunable correctly
would result in complexity that would outweigh any foreseeable benefits.
For example, disabling the feature globally would involve deleting all
colocation groups one by one while ensuring no placement decisions are
made during the process.

Remove the tunable. Adding or removing a task from a colocation group is
still possible and so we're not losing functionality.

Change-Id: I4cb8bcdbee98d3bdd168baacbac345eca9ea8879
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>