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/set_abort_message.h b/libc/include/android/set_abort_message.h
index 2be01a9..35867ac 100644
--- a/libc/include/android/set_abort_message.h
+++ b/libc/include/android/set_abort_message.h
@@ -43,9 +43,7 @@
  * This is meant for use by libraries that deliberately abort so that they can
  * provide an explanation. It is used within bionic to implement assert() and
  * all FORTIFY/fdsan aborts.
- *
- * Available since API level 21.
  */
-void android_set_abort_message(const char* _Nullable __msg) __INTRODUCED_IN(21);
+void android_set_abort_message(const char* _Nullable __msg);
 
 __END_DECLS