Kernel+LibC: Remove sys$donate()

This was an old SerenityOS-specific syscall for donating the remainder
of the calling thread's time-slice to another thread within the same
process.

Now that Threading::Lock uses a pthread_mutex_t internally, we no
longer need this syscall, which allows us to get rid of a surprising
amount of unnecessary scheduler logic. :^)

Kernel/API/Syscall.h

@@ -113,7 +113,6 @@ namespace Kernel {
     S(setsockopt)                 \
     S(create_thread)              \
     S(gettid)                     \
-    S(donate)                     \
     S(rename)                     \
     S(ftruncate)                  \
     S(exit_thread)                \