fix(cdk): jsx support conflicts with React usage (#884)

Fixes #830

BREAKING CHANGE

The CDK is no longer shipped with built-in support for JSX.
You can still use JSX but you will have to manually configure it.

Author: Elad Ben-Israel

Diff for: packages/@aws-cdk/cdk/lib/core/jsx.ts

@@ -1,7 +1,6 @@
 export * from './core/construct';
 export * from './core/tokens';
 export * from './core/tag-manager';
-export * from './core/jsx';
 
 export * from './cloudformation/cloudformation-json';
 export * from './cloudformation/cloudformation-token';