Remove support for TrxBoundaryMatcher

For the general case, regex matching simply will not do. The
regex becomes so hairy so it turns write-only, i.e. unmaintainable.
Regex matching is also slower than a handwritten custom parser.

server/core/maxscale/query_classifier.h

@@ -20,7 +20,6 @@ MXS_BEGIN_DECLS
 typedef enum qc_trx_parse_using
 {
     QC_TRX_PARSE_USING_QC,     /**< Use the query classifier. */
-    QC_TRX_PARSE_USING_REGEX,  /**< Use regexp mathing. */
     QC_TRX_PARSE_USING_PARSER, /**< Use custom parser. */
 } qc_trx_parse_using_t;