refactor(Observable.if): remove ts hacks from Observable

BREAKING CHANGE: TypeScript users using `Observable.if` will have to cast `Observable` as any to get to `if`. It is a better idae to just use `iif` directly via `import { iif } from 'rxjs';`

src/add/observable/if.ts

@@ -1,4 +1,5 @@
 import { Observable } from '../../internal/Observable';
 import { iif } from '../../internal/observable/iif';
 
-Observable.if = iif;
+//tslint:disable-next-line:no-any TypeScript doesn't like `if`
+(Observable as any).if = iif;