lwt: Don't disable migration prio invoking BPF.

There is no need to explicitly disable migration if bottom halves are
also disabled. Disabling BH implies disabling migration.

Remove migrate_disable() and rely solely on disabling BH to remain on
the same CPU.

Cc: bpf@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>