Remove leftover NUI code paths

[JSUYA]

#184 removed the NUI/Dali embedding support, but a few code paths that only served the NUI image view were left behind. Every remaining view is a TizenWindow, so these paths are unreachable. Remove them.

• tizen_input_method_context_tcore.*: Remove HandleNuiKeyEvent(). It was carried into the tizen-core input method context (Support tizen-core window #195) after Remove FlutterView (NUI/Dali embedding) support #184, and was always compiled out because NUI_SUPPORT is no longer defined.
• tizen_renderer_egl.cc: Remove the tbm dummy display fallback, which was used when the view had no native display (the NUI image view). Fail early if no display is given instead of creating a dummy display that is never released.
• flutter_tizen_view.cc: Remove the non-window branch from SendInitialGeometry().

No behavior change for window-based views.

[chatgpt-codex-connector]

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[JSUYA]

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Um... I still have remaining Codex usage and plenty of credits. Codex is acting up, though; it is currently known that the codex review bot for pull requests created from forked git repo isn't working properly.

[JSUYA]

The current CI build error requires #203.