freezer: don't distinguish nosig tasks on thaw

There's no point in thawing nosig tasks before others.  There's no
ordering requirement between the two groups on thaw, which the staged
thawing can't guarantee anyway.  Simplify thaw_processes() by removing
the distinction and collapsing thaw_tasks() into thaw_processes().
This will help further updates to freezer.

Signed-off-by: Tejun Heo <tj@kernel.org>