* [react-bootstrap] remove incorrect exports
several sub-components are exported in index.d.ts, when they are not
in the library itself.
should fail:
`import { DropdownMenu } from 'react-bootstrap';`
should succeed:
`import { Dropdown } from 'react-bootstrap';`
`import DropdownMenu from 'react-bootstrap/lib/DropdownMenu';`
* [react-bootstrap] fix Dropdown.Menu typing
DropdownMenu uses `export =`, so the import must use require, `import *`.
this matches the pattern in Carousel, FormControl, etc.
