timers/migration: Remove dead code handling idle CPU checking for remote timers

timers/migration: Remove dead code handling idle CPU checking for remote timers

Idle migrators don't walk the whole tree in order to find out if there
are timers to migrate because they recorded the next deadline to be
verified within a single check in tmigr_requires_handle_remote().

Remove the related dead code and data.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251024132536.39841-7-frederic@kernel.org