Update .placeholder() mixin call

In less/mixins.less this mixin is being created with @placeholderText
color as default, which is actually set as @placeholderText: @grayLight
in less/variables.less so it's redundant to make call like this:

.placeholder(@grayLight);

less/forms.less

@@ -344,7 +344,7 @@ select:focus:required:invalid {
 }
 
 // Placeholder text gets special styles; can't be bundled together though for some reason
-.placeholder(@grayLight);
+.placeholder();