build: Stop using xmllint to validate protocol files

Same reason as commit cd31275f from weston:

    The scanner needs to be good enough.  If it crashes or fails to report
    invalid input, that needs to get fixed.

configure.ac

@@ -87,9 +87,6 @@ fi
 AC_PATH_PROG(XSLTPROC, xsltproc)
 AM_CONDITIONAL([HAVE_XSLTPROC], [test "x$XSLTPROC" != "x"])
 
-AC_PATH_PROG(XMLLINT, xmllint)
-AM_CONDITIONAL([HAVE_XMLLINT], [test "x$XMLLINT" != "x"])
-
 AC_MSG_CHECKING([for docbook manpages stylesheet])
 MANPAGES_STYLESHEET=http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
 AC_PATH_PROGS_FEATURE_CHECK([XSLTPROC_TMP], [xsltproc],

protocol/Makefile.am

 dist_pkgdata_DATA = wayland.xml wayland.dtd
-
-if HAVE_XMLLINT
-.PHONY: validate
-
-.%.xml.valid: %.xml
-	$(AM_V_GEN)$(XMLLINT) --noout --dtdvalid $(srcdir)/wayland.dtd $^ > $@
-
-validate: .wayland.xml.valid
-
-all-local: validate
-
-CLEANFILES = .wayland.xml.valid
-endif