refactor(axis): fix lint error (#2350)

src/axis.js

@@ -475,15 +475,11 @@ c3_axis_fn.getId = function getId(id) {
     return id in config.data_axes ? config.data_axes[id] : 'y';
 };
 c3_axis_fn.getXAxisTickFormat = function getXAxisTickFormat() {
-    // #2251 previously set any negative values to a whole number, 
+    // #2251 previously set any negative values to a whole number,
     // however both should be truncated according to the users format specification
     var $$ = this.owner, config = $$.config;
-    let format = ($$.isTimeSeries()) 
-        ? $$.defaultAxisTimeFormat 
-        : ($$.isCategorized()) 
-            ? $$.categoryName 
-            : function (v) { return v; };
-            
+    let format = ($$.isTimeSeries()) ? $$.defaultAxisTimeFormat : ($$.isCategorized()) ? $$.categoryName : function (v) { return v; };
+
     if (config.axis_x_tick_format) {
         if (isFunction(config.axis_x_tick_format)) {
             format = config.axis_x_tick_format;