xbps_configure_pkg: simplify and remove "version" arg (API change).

The "version" argument (being optional) clearly is not needed and it
can be fully removed. This simplifies the internal implementation as well
as the API.

include/xbps_api.h

@@ -603,7 +603,6 @@ struct xbps_handle *xbps_handle_get(void);
  * Configure (or force reconfiguration of) a package.
  *
  * @param[in] pkgname Package name to configure.
- * @param[in] version Package version (<b>optional</b>).
  * @param[in] check_state Set it to true to check that package is
  * in unpacked state.
  * @param[in] update Set it to true if this package is being updated.
@@ -612,7 +611,6 @@ struct xbps_handle *xbps_handle_get(void);
  * @return 0 on success, otherwise an errno value.
  */
 int xbps_configure_pkg(const char *pkgname,
-		       const char *version,
 		       bool check_state,
 		       bool update,
 		       bool flush);