[configure] We have a local copy of the header file, remove the check

We do have a local copy, there is no need to check for an
external version of that header file as it will not be used.

diff --git a/configure.in b/configure.in
index 6a6d3a9..edcbe3b 100644
--- a/configure.in
+++ b/configure.in
@@ -13,7 +13,6 @@
 
 dnl checks for header files
 AC_HEADER_STDC
-AC_CHECK_HEADERS(mISDNif.h)
 
 dnl Checks for typedefs, structures and compiler characteristics