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/poll.h b/libc/include/poll.h
index e3a9039..6bdc886 100644
--- a/libc/include/poll.h
+++ b/libc/include/poll.h
@@ -58,10 +58,8 @@
  *
  * Returns the number of ready file descriptors on success, 0 for timeout,
  * and returns -1 and sets `errno` on failure.
- *
- * Available since API level 28.
  */
-int ppoll(struct pollfd* _Nullable __fds, nfds_t __count, const struct timespec* _Nullable __timeout, const sigset_t* _Nullable __mask) __INTRODUCED_IN(21);
+int ppoll(struct pollfd* _Nullable __fds, nfds_t __count, const struct timespec* _Nullable __timeout, const sigset_t* _Nullable __mask);
 
 /**
  * Like ppoll() but allows setting a signal mask with RT signals even from a 32-bit process.