Remove remaingin zeroLine* references (#6728)

Author: kurkle

samples/charts/scatter/multi-axis.html

@@ -97,9 +97,6 @@
 				scales: {
 					xAxes: [{
 						position: 'bottom',
-						gridLines: {
-							zeroLineColor: 'rgba(0,0,0,1)'
-						}
 					}],
 					yAxes: [{
 						type: 'linear', // only linear but allow scale type registration. This allows extensions to exist solely for log scale for instance

test/fixtures/core.scale/tick-drawing.json

@@ -46,8 +46,7 @@
                     "gridLines":{
                         "drawOnChartArea": false,
                         "drawBorder": false,
-                        "color": "rgba(0, 0, 0, 1)",
-                        "zeroLineColor": "rgba(0, 0, 0, 1)"
+                        "color": "rgba(0, 0, 0, 1)"
                     }
                 }, {
                     "type": "linear",
@@ -61,8 +60,7 @@
                     "gridLines":{
                         "drawOnChartArea": false,
                         "drawBorder": false,
-                        "color": "rgba(0, 0, 0, 1)",
-                        "zeroLineColor": "rgba(0, 0, 0, 1)"
+                        "color": "rgba(0, 0, 0, 1)"
                     }
                 }]
             }