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/signalfd.h b/libc/include/sys/signalfd.h
index f669cc8..2be9bdc 100644
--- a/libc/include/sys/signalfd.h
+++ b/libc/include/sys/signalfd.h
@@ -45,10 +45,8 @@
  * file descriptor for reading signal events.
  *
  * Returns the file descriptor on success, and returns -1 and sets `errno` on failure.
- *
- * Available since API level 18.
  */
-int signalfd(int __fd, const sigset_t* _Nonnull __mask, int __flags) __INTRODUCED_IN(18);
+int signalfd(int __fd, const sigset_t* _Nonnull __mask, int __flags);
 
 /**
  * Like signalfd() but allows setting a signal mask with RT signals even from a 32-bit process.