Remove unused keyboard shortcuts code
Summary:
`PHUIPropertyListView` defines `setHasKeyboardShortcuts()` using `AphrontKeyboardShortcutsAvailableView` setting the CSS class `keyboard-shortcuts-available` defined in `phui-property-list-view.css` and `phui-property-list-view.css`, the latter being the only consumer of the icon `key_question.png`.
Searching the codebase for `asKeyboardShortcuts` there are zero callers as `src/applications/differential/view/DifferentialRevisionDetailView.php` got deleted in https://we.phorge.it/rP148a50e48be146b88036032d1cad772d2627da28. Thus remove that unused code/files.
Note that this is unrelated to `setEnableKeyboardShortcuts()` in `PHUIPagerView`.
Test Plan: Grep the codebase for `asKeyboardShortcuts`
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26005
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