Remove unsupported feature

lib/createPageContext.js

@@ -7,8 +7,6 @@ function createPageContext() {
   return {
     // The standard class name generator.
     generateClassName: createGenerateClassName(),
-    // This is needed in order to deduplicate the injection of CSS in the page.
-    sheetsManager: new Map(),
     // This is needed in order to inject the critical CSS.
     sheetsRegistry: new SheetsRegistry(),
     theme,

lib/withRoot.jsx

@@ -29,7 +29,6 @@ function withRoot(Component) {
       // context.
       return (
         <MuiThemeProvider
-          sheetsManager={pageContext.sheetsManager}
           theme={pageContext.theme}>
           {/* CssBaseline kickstart an elegant, consistent, and simple baseline to */}
           {/* build upon. */}