Remove fontSize comments

Not really as useful when we aren't publishing the config, plus we
don't list them in the docs either.

defaultTheme.js

@@ -142,15 +142,15 @@ module.exports = function() {
       mono: ['Menlo', 'Monaco', 'Consolas', 'Liberation Mono', 'Courier New', 'monospace'],
     },
     fontSize: {
-      xs: '.75rem', // 12px
-      sm: '.875rem', // 14px
-      base: '1rem', // 16px
-      lg: '1.125rem', // 18px
-      xl: '1.25rem', // 20px
-      '2xl': '1.5rem', // 24px
-      '3xl': '1.875rem', // 30px
-      '4xl': '2.25rem', // 36px
-      '5xl': '3rem', // 48px
+      xs: '.75rem',
+      sm: '.875rem',
+      base: '1rem',
+      lg: '1.125rem',
+      xl: '1.25rem',
+      '2xl': '1.5rem',
+      '3xl': '1.875rem',
+      '4xl': '2.25rem',
+      '5xl': '3rem',
     },
     fontWeight: {
       hairline: 100,