Remove unused isc_mem_createx() function

The isc_mem_createx() function was only used in the tests to eliminate using the
default flags (which as of writing this commit message was ISC_MEMFLAG_INTERNAL
and ISC_MEMFLAG_FILL).  This commit removes the isc_mem_createx() function from
the public API.

lib/isc/tests/Makefile.in

@@ -16,7 +16,10 @@ VERSION=@BIND9_VERSION@
 @BIND9_MAKE_INCLUDES@
 
 CINCLUDES =	-I. -Iinclude ${ISC_INCLUDES} \
-		${OPENSSL_CFLAGS} @CMOCKA_CFLAGS@
+		${OPENSSL_CFLAGS} @CMOCKA_CFLAGS@ \
+		${JSON_C_CFLAGS} \
+		${LIBXML2_CFLAGS} \
+		${ZLIB_CFLAGS}
 CDEFINES =	-DTESTS="\"${top_builddir}/lib/isc/tests/\""
 
 ISCLIBS =	../libisc.@A@ ${OPENSSL_LIBS} ${JSON_C_LIBS} ${LIBXML2_LIBS}