Revert "<sched.h> should offer both __sched_priority and sched_priority."

This was only ever added to keep strace compiling, but strace has
since (actually, prior) fixed their source to use the less ugly name,
so we don't need this anymore (good riddance, #define!).

This reverts commit bbb34f9536db8234b0b43bd5333256fa4accc50e.
Merge "Revert "<sched.h> should offer both __sched_priority and sched_priority.""