There doesn't seem to be an especially clean way. One simple approach is to set the Legend's Width to zero using LegendStyle:
<charting:Chart>
<charting:Chart.LegendStyle>
<Style TargetType="datavis:Legend">
<Setter Property="Width" Value="0" />
</Style>
</charting:Chart.LegendStyle>
A more drastic approach is to replace the ControlTemplate with one that does not include a Legend:
<charting:Chart>
<charting:Chart.Template>
<ControlTemplate TargetType="{x:Type charting:Chart}">
<Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<datavis:Title Content="{TemplateBinding Title}" Style="{TemplateBinding TitleStyle}" />
<chartingprimitives:EdgePanel Name="ChartArea" Style="{TemplateBinding ChartAreaStyle}" Grid.Row="1" Margin="0,15,0,15">
<Grid Panel.ZIndex="-1" Style="{TemplateBinding PlotAreaStyle}" />
<Border Panel.ZIndex="10" BorderBrush="#FF919191" BorderThickness="1" />
</chartingprimitives:EdgePanel>
</Grid>
</Border>
</ControlTemplate>
</charting:Chart.Template>