headers: Remove now unused `AC_PROG_SED`

It was used to process _mingw_ddk.h.in, which has been removed.

Signed-off-by: LIU Hao <lh_mouse@126.com>

mingw-w64-headers/configure.ac

diff --git a/mingw-w64-headers/configure.ac b/mingw-w64-headers/configure.ac
index 3211fbd..186e989 100644
--- a/mingw-w64-headers/configure.ac
+++ b/mingw-w64-headers/configure.ac
@@ -12,9 +12,6 @@
 
 AC_CANONICAL_HOST
 
-# Checks for programs.
-AC_PROG_SED
-
 # Search for a usable widl.  If PATH is given, search there.  Otherwise, search
 # in $PATH.  In all cases, search for a canonicalized name first ($host-widl),
 # followed by a non-canonicalized name (widl).  If these fail, error out.