Revert "ARC: [SMP] reset sw-asid before getting new context"

With mm_cpumask being sticky now, we don't need this.

Arguably we can optimize the tlb flush IPIs for single threaded process
migratign across cores, that's a seperate change.

This reverts commit b7efb6332e5bb80a1c585981404c21a1d9335900.