Remove obsolete libxml2 code

LIBXML2_NEW_BUFFER is always defined since libxml2 2.9.0.
That's the minimum version PHP requires, so it will always be defined.

Author: ndossche

ext/dom/documenttype.c

@@ -175,11 +175,7 @@ zend_result dom_documenttype_internal_subset_read(dom_object *obj, zval *retval)
 				xmlNodeDumpOutput (buff, NULL, cur, 0, 0, NULL);
 				xmlOutputBufferFlush(buff);
 
-#ifdef LIBXML2_NEW_BUFFER
 				smart_str_appendl(&ret_buf, (const char *) xmlOutputBufferGetContent(buff), xmlOutputBufferGetSize(buff));
-#else
-				smart_str_appendl(&ret_buf, (char *) buff->buffer->content, buff->buffer->use);
-#endif
 
 				(void)xmlOutputBufferClose(buff);
 			}

ext/dom/node.c

@@ -1891,17 +1891,9 @@ static void dom_canonicalization(INTERNAL_FUNCTION_PARAMETERS, int mode) /* {{{
 		RETVAL_FALSE;
 	} else {
 		if (mode == 0) {
-#ifdef LIBXML2_NEW_BUFFER
 			ret = xmlOutputBufferGetSize(buf);
-#else
-			ret = buf->buffer->use;
-#endif
 			if (ret > 0) {
-#ifdef LIBXML2_NEW_BUFFER
 				RETVAL_STRINGL((char *) xmlOutputBufferGetContent(buf), ret);
-#else
-				RETVAL_STRINGL((char *) buf->buffer->content, ret);
-#endif
 			} else {
 				RETVAL_EMPTY_STRING();
 			}

ext/simplexml/simplexml.c

@@ -1432,13 +1432,8 @@ PHP_METHOD(SimpleXMLElement, asXML)
 
 		xmlNodeDumpOutput(outbuf, (xmlDocPtr) sxe->document->ptr, node, 0, 0, (const char *) ((xmlDocPtr) sxe->document->ptr)->encoding);
 		xmlOutputBufferFlush(outbuf);
-#ifdef LIBXML2_NEW_BUFFER
 		return_content = (char *)xmlOutputBufferGetContent(outbuf);
 		return_len = xmlOutputBufferGetSize(outbuf);
-#else
-		return_content = (char *)outbuf->buffer->content;
-		return_len = outbuf->buffer->use;
-#endif
 		if (!return_content) {
 			RETVAL_FALSE;
 		} else {