Removed performance section from stylesheet as no longer true (#629)

See facebook@a8e3c7f
`StyleSheet.create` is now an identity function, the `ReactNativePropRegistry` (`StyleSheetRegistry`) was removed.

Author: danilobuerger

docs/stylesheet.md

@@ -37,11 +37,6 @@ Code quality:
 * By moving styles away from the render function, you're making the code easier to understand.
 * Naming the styles is a good way to add meaning to the low level components in the render function.
 
-Performance:
-
-* Making a stylesheet from a style object makes it possible to refer to it by ID instead of creating a new style object every time.
-* It also allows to send the style only once through the bridge. All subsequent uses are going to refer an id (not implemented yet).
-
 ### Methods
 
 * [`setStyleAttributePreprocessor`](stylesheet.md#setstyleattributepreprocessor)