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/statvfs.h b/libc/include/sys/statvfs.h
index d81f836..46fbea5 100644
--- a/libc/include/sys/statvfs.h
+++ b/libc/include/sys/statvfs.h
@@ -93,25 +93,21 @@
  * queries filesystem statistics for the given path.
  *
  * Returns 0 on success, and returns -1 and sets `errno` on failure.
- *
- * Available since API level 19.
  */
-int statvfs(const char* _Nonnull __path, struct statvfs* _Nonnull __buf) __INTRODUCED_IN(19);
+int statvfs(const char* _Nonnull __path, struct statvfs* _Nonnull __buf);
 
 /**
  * [fstatvfs(3)](http://man7.org/linux/man-pages/man3/fstatvfs.3.html)
  * queries filesystem statistics for the given file descriptor.
  *
  * Returns 0 on success, and returns -1 and sets `errno` on failure.
- *
- * Available since API level 19.
  */
-int fstatvfs(int __fd, struct statvfs* _Nonnull __buf) __INTRODUCED_IN(19);
+int fstatvfs(int __fd, struct statvfs* _Nonnull __buf);
 
 /** Equivalent to statvfs() . */
-int statvfs64(const char* _Nonnull __path, struct statvfs64* _Nonnull __buf) __INTRODUCED_IN(21);
+int statvfs64(const char* _Nonnull __path, struct statvfs64* _Nonnull __buf);
 
 /** Equivalent to fstatvfs(). */
-int fstatvfs64(int __fd, struct statvfs64* _Nonnull __buf) __INTRODUCED_IN(21);
+int fstatvfs64(int __fd, struct statvfs64* _Nonnull __buf);
 
 __END_DECLS