remove flow type, the file are not meant to be compiled

[ignatiusreza]

[jaruesink]

@rt2zz this is blocking me from adding this to my project, because create react app doesn't strip types. Same issue as #22

index.js:1452 ./node_modules/redux-persist-crosstab/index.js
SyntaxError: /Users/jruesink/Documents/projects/MGMW/mgmw-app/node_modules/redux-persist-crosstab/index.js: Unexpected token (3:12)

  1 | // @flow
  2 | import { KEY_PREFIX, REHYDRATE } from 'redux-persist/lib/constants';
> 3 | import type { PersistConfig } from 'redux-persist/es/types';
    |             ^
  4 | import type { Store } from 'redux';
  5 | 
  6 | type CrosstabConfig = {