Remove __INTRODUCED_IN for obsolete API levels.

The NDK only supports >= 16, so remove anything older than that to avoid
giving the misleading impression that such old targets are still
supported.

(This change doesn't touch <unistd.h>. I'll follow up with that once the
outstanding FORTIFY changes to that file are in.)

Test: builds
Change-Id: I6cc6ecdb99fe228a4afa71f78e5fd45309ba9786
diff --git a/libc/include/sys/system_properties.h b/libc/include/sys/system_properties.h
index f469599..a2e1923 100644
--- a/libc/include/sys/system_properties.h
+++ b/libc/include/sys/system_properties.h
@@ -43,7 +43,7 @@
 /*
  * Sets system property `name` to `value`, creating the system property if it doesn't already exist.
  */
-int __system_property_set(const char* __name, const char* __value) __INTRODUCED_IN(12);
+int __system_property_set(const char* __name, const char* __value);
 
 /*
  * Returns a `prop_info` corresponding system property `name`, or nullptr if it doesn't exist.