MXS-1929: Take SFilterDef into use

The service now uses a std::vector<SFilterDef> to store the filters it
uses. Most internal parts deal with the SFilterDef but debugcmd.cc still
moves raw pointers around (needs to be changed).

include/maxscale/filter.h

@@ -225,16 +225,6 @@ typedef struct mxs_filter_def
 {
 } MXS_FILTER_DEF;
 
-/**
- * Lookup a filter definition using the unique section name in
- * the configuration file.
- *
- * @param name The name of a filter.
- *
- * @return A filter definition or NULL if not found.
- */
-MXS_FILTER_DEF *filter_def_find(const char *name);
-
 /**
  * Get the name of a filter definition. This corresponds to
  * to a filter section in the configuration file.