cpumask: drop cpumask_assign_cpu()

Commit decde1fa ("cpumask: Add assign cpu") was merged bypassing
cpumasks reviewers. It adds atomic and non-atomic cpumask_assign_cpu()
helpers.

In the same merge window, commit 5c563ee9 ("cpumask: introduce
assign_cpu() macro") added the same functionality. So now we have it
duplicated.

__cpumask_assign_cpu() has never been used since introducing, and because
this series reworks the only user of cpumask_assign_cpu(), both functions
become a dead code.

Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>