Revert "[TextField] Fix blurry text on label (mui#19547)"

Closes mui#19677

This reverts commit 9c3904d.

Author: piotros

packages/material-ui/src/FormControl/FormControl.js

@@ -18,7 +18,6 @@ export const styles = {
     padding: 0,
     margin: 0,
     border: 0,
-    zIndex: 0, // Fix blur label text issue
     verticalAlign: 'top', // Fix alignment issue on Safari.
   },
   /* Styles applied to the root element if `margin="normal"`. */