configure.ac: remove the AC_CHECK_HEADERS checks

some of these headers aren't used in the protobuf-c code base any more,
and in any case the results of these checks (the HAVE_*_H defines in
config.h) are not actually used anywhere and the absence of any of these
headers doesn't cause configure to fail, so just delete these useless
checks.

configure.ac

@@ -14,16 +14,6 @@ LT_INIT
 AC_CONFIG_HEADERS(config.h)
 AC_CONFIG_FILES([Makefile protobuf-c/libprotobuf-c.pc])
 
-AC_CHECK_HEADERS(alloca.h)
-AC_CHECK_HEADERS(inttypes.h)
-AC_CHECK_HEADERS(io.h)
-AC_CHECK_HEADERS(malloc.h)
-AC_CHECK_HEADERS(sys/poll.h)
-AC_CHECK_HEADERS(sys/select.h)
-AC_CHECK_HEADERS(sys/uio.h)
-AC_CHECK_HEADERS(unistd.h)
-AC_CHECK_HEADERS(winsock.h)
-
 PKG_PROG_PKG_CONFIG
 if test -n "$PKG_CONFIG"; then
     # Horrible hack for systems where the pkg-config install directory is simply wrong!