Kernel: Remove SYS_SLEEP and cleanup SYS_NANOSLEEP

`sleep` is now implemented in terms of `nanosleep`. `nanosleep` is now
more precise and handles overflow when calculating wakeup time. I don't
think anything was depending on this, but I could see a program sleeping
for max time to block until signal.

kernel/include/kernel/Process.h

@@ -68,7 +68,6 @@ namespace Kernel
 		BAN::ErrorOr<long> sys_exec(const char* path, const char* const* argv, const char* const* envp);
 
 		BAN::ErrorOr<long> sys_wait(pid_t pid, int* stat_loc, int options);
-		BAN::ErrorOr<long> sys_sleep(int seconds);
 		BAN::ErrorOr<long> sys_nanosleep(const timespec* rqtp, timespec* rmtp);
 		BAN::ErrorOr<long> sys_setitimer(int which, const itimerval* value, itimerval* ovalue);