Don't restrict answer key shortcuts

Qt seems to handle invalid input gracefully, and the previous approach
prevented things like umlauts or keys with modifiers.

https://forums.ankiweb.net/t/bug-cannot-set-answer-key-to-german-umlaut/37091

qt/aqt/preferences.py

@@ -70,11 +70,6 @@ class Preferences(QDialog):
                 line_edit.textChanged,
                 functools.partial(self.mw.pm.set_answer_key, ease),
             )
-            line_edit.setValidator(
-                QRegularExpressionValidator(
-                    QRegularExpression(r"^[a-z0-9\]\[=,./;\'\\-]$")
-                )
-            )
             line_edit.setPlaceholderText(tr.preferences_shortcut_placeholder())
 
     def accept(self) -> None: