Remove MDX Conf 2020 banner (#1530)

Sorry that banner for a conference in 2020 was always annoying me, I have no idea if this is the right way, but I hope so!

Author: ChrisChinchilla

packages/gatsby-theme-mdx/src/components/layout.js

@@ -30,32 +30,6 @@ const styles = (
   />
 )
 
-const MDXConf = () => (
-  <div
-    css={css({
-      bg: '#fcb42d',
-      boxSizing: 'border-box'
-    })}
-  >
-    <Link
-      css={css({
-        color: 'text',
-        textDecoration: 'none',
-        fontWeight: 'bold',
-        fontSize: [2, 3, 3],
-        width: '100%',
-        display: 'flex',
-        p: 3
-      })}
-      to="/conf"
-    >
-      <span aria-label="tada">🎉</span>
-      <span css={css({ml: 2})}>MDX Conf &mdash; August 24th, 2020</span>
-      <span css={css({ml: 'auto'})}>&rarr;</span>
-    </Link>
-  </div>
-)
-
 const Root = props => (
   <div
     {...props}
@@ -195,7 +169,6 @@ export default props => {
       <ThemeProvider theme={theme} components={components}>
         <SkipNavLink />
         {styles}
-        <MDXConf />
         <Root>
           <Overlay open={menuOpen} onClick={closeMenu} />
           <Header toggleMenu={toggleMenu} dark={dark} setDark={setDark} />