qt: Remove focus indication by text selection

* qt/accessibility.cpp, qt/accessibility.h (selectLabelText): Remove.
* qt/pinentryconfirm.cpp, qt/pinentryconfirm.h (focusNextPrevChild):
Remove.
* qt/pinentrydialog.cpp: Remove class TextLabel.
(PinEntryDialog::PinEntryDialog): Use QLabel for all labels.

Now that we have added proper keyboard focus indication for (text) labels
we can remove the crude indication by selecting all text. In particular,
this prevents potentially confusing text selection notifications by
assistive tools.

• GnuPG-bug-id: T5863