sokol_app.h emsc: remove hardwired key event consumption table in _sapp_emsc_key_cb()

[floooh]

...this is causing all sorts of problems when there are other page elements outside the wasm app which need to receive keyboard input.

Instead offload the responsibility to the application by calling sapp_consume_event() from the event callback.

Related issues and PRs: #822 , #800, #801