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. :^)

Userland/Libraries/LibC/unistd.cpp

@@ -695,12 +695,6 @@ int gettid()
     return cached_tid;
 }
 
-int donate(int tid)
-{
-    int rc = syscall(SC_donate, tid);
-    __RETURN_WITH_ERRNO(rc, rc, -1);
-}
-
 void sysbeep()
 {
     syscall(SC_beep);

Userland/Libraries/LibC/unistd.h

@@ -40,7 +40,6 @@ void dump_backtrace();
 int fsync(int fd);
 void sysbeep();
 int gettid();
-int donate(int tid);
 int getpagesize();
 pid_t fork();
 pid_t vfork();