remove flow check in electron app.js (#26184)

When we were upgrading flow in
https://github.com/facebook/react/commit/6ddcbd4f96cb103de3978617a53c200baf5b546c
we added `$FlowFixMe` for some parameters in this file. However, this
file is not compiled at all, and the `:` syntax breaks the code.

This PR removes the flow check in this file