changed method call to be consistent with similar calls

src/main/java/com/xeiam/xchart/Chart.java

@@ -191,12 +191,12 @@ public class Chart {
 
   public void setXAxisTitle(String title) {
 
-    this.axisPair.xAxis.setAxisTitle(title);
+    this.axisPair.xAxis.axisTitle.setText(title);
   }
 
   public void setYAxisTitle(String title) {
 
-    this.axisPair.yAxis.setAxisTitle(title);
+    this.axisPair.yAxis.axisTitle.setText(title);
   }
 
   // ChartPart visibility ////////////////////////////////

src/main/java/com/xeiam/xchart/internal/chartpart/Axis.java

@@ -129,11 +129,6 @@ public class Axis implements IChartPart {
     return axisTitle;
   }
 
-  public void setAxisTitle(String title) {
-
-    this.axisTitle.setText(title);
-  }
-
   protected void setAxisTitle(AxisTitle axisTitle) {
 
     this.axisTitle = axisTitle;