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/android/dlext.h b/libc/include/android/dlext.h
index f216aab..a5061c7 100644
--- a/libc/include/android/dlext.h
+++ b/libc/include/android/dlext.h
@@ -177,11 +177,8 @@
  * Opens the given library. The `__filename` and `__flags` arguments are
  * the same as for [dlopen(3)](http://man7.org/linux/man-pages/man3/dlopen.3.html),
  * with the Android-specific flags supplied via the `flags` member of `__info`.
- *
- * Available since API level 21.
  */
-void* _Nullable android_dlopen_ext(const char* _Nullable __filename, int __flags, const android_dlextinfo* _Nullable __info)
-  __INTRODUCED_IN(21);
+void* _Nullable android_dlopen_ext(const char* _Nullable __filename, int __flags, const android_dlextinfo* _Nullable __info);
 
 __END_DECLS