Fix compiler warnings in ewk/efl_webview_app/app.c

Fix compiler warnings in ewk/efl_webview_app/app.c

Fixes following warnings in app.c:
> 'prompt_entry' defined but not used [-Wunused-variable]
> '__scroll_down_cb' defined but not used [-Wunused-function]
> '__scroll_up_cb' defined but not used [-Wunused-function]
> '__scroll_left_cb' defined but not used [-Wunused-function]
> '__scroll_right_cb' defined but not used [-Wunused-function]

Bug: http://web.sec.samsung.net/bugzilla/show_bug.cgi?id=15559

Reviewed by: a1.gomes, sns.park

Change-Id: Ic9c7ae0109e3b2cb75ecdab0b05f644590ce28d3
Signed-off-by: y0.kim <y0.kim@samsung.com>