[Slider]: Remove visual zero state from thumb

The spec does not show any visual difference between thumbs at zero and
non-zero values. Zero values now allow different thumb colors depending
on state (i.e. enabled sliders appeared to be disabled).

packages/material-ui-lab/src/Slider/Slider.js

@@ -123,18 +123,6 @@ export const styles = theme => {
         height: 9,
         backgroundColor: colors.disabled,
       },
-      '&$zero': {
-        border: `2px solid ${colors.disabled}`,
-        backgroundColor: 'transparent',
-      },
-      '&$focused$zero': {
-        border: `2px solid ${colors.focused}`,
-        backgroundColor: fade(colors.focused, 0.34),
-        boxShadow: `0px 0px 0px 9px ${fade(colors.focused, 0.34)}`,
-      },
-      '&$activated$zero': {
-        border: `2px solid ${colors.focused}`,
-      },
       '&$jumped': {
         width: 17,
         height: 17,