Fix ewk_view_text_selection_clear

Fix ewk_view_text_selection_clear

When ewk_view_text_selection_clear() is called, selection in renderer
is cleared. But we should also clear selected text in browser.
Because we can get selected text in RWHVBase by using
ewk_view_text_selection_text_get().

This patch makes clear selected text in RWHVBase when
ewk_view_text_selection_clear() is called.
And remove unnecessary variable in EWebView.

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=13560
Reviewed by: Antonio Gomes, Jaesik Chang

Change-Id: Ib9ae750ed6361d9eb0e7f8b42c08fa59edd35511
Signed-off-by: Sanghyup Lee <sh53.lee@samsung.com>