scheduler/EDF: Use unprotected insert and extract

Interrupts are disabled by the caller _Thread_Change_priority() or
_Thread_Set_transient() or directly in the scheduler operation.  Thus
there is no need to use protected variants.
score/rbtree: Delete protected operations

The user of the red-black tree container must now ensure that at most
one thread at once can access an instance.
score/rbtree: Remove "unprotected" from API