configure.ac: remove explicit default path for libxml2

OpenEmbedded needs to cross-compile rrdtool, and historically has applied this patch locally because the implementation of EX_CHECK_ALL unconditionally added -I/usr/include.  Since the correct include path is automatically provided by a successful pkg-config invocation there is no need to set it explicitly.

(Commit cef27b4d0de373a6d5a865fd156a474d29179e5b changed the context so the patch OE's been using no longer applies; I figured it might as well be fixed upstream instead.)