fix(preact): Remove fallback for PureComponent since it's now support…

…ed by preact-compat

As a bonus, this fixes the warning generated when running tests:
"export 'PureComponent' (imported as '_React_') was not found in 'react'
preactjs/preact-compat#139

src/imports.js

@@ -6,7 +6,6 @@ import * as types from 'prop-types';
 export const React = {
   createElement: _React_.createElement || _React_.h
 };
-export const PureComponent = _React_.PureComponent || _React_.Component;
 export const PropTypes = types;
 export const render = ReactDOM.render;