Remove the always-true __INTRODUCED_IN() annotations.

The NDK no longer supports API levels earlier than 21.

This *doesn't* include <ctype.h> because I have a separate change
rewriting that (that's blocked on the upcoming libc++ update).

Test: treehugger
Change-Id: I53e915f27011dfc0513e0c78d8799377e183ceca
diff --git a/libc/include/sys/wait.h b/libc/include/sys/wait.h
index e6fb855..5208366 100644
--- a/libc/include/sys/wait.h
+++ b/libc/include/sys/wait.h
@@ -39,7 +39,7 @@
 
 pid_t wait(int* _Nullable __status);
 pid_t waitpid(pid_t __pid, int* _Nullable __status, int __options);
-pid_t wait4(pid_t __pid, int* _Nullable __status, int __options, struct rusage* _Nullable __rusage) __INTRODUCED_IN(18);
+pid_t wait4(pid_t __pid, int* _Nullable __status, int __options, struct rusage* _Nullable __rusage);
 
 /* Posix states that idtype_t should be an enumeration type, but
  * the kernel headers define P_ALL, P_PID and P_PGID as constant macros