qt: Do not set accessible description of buttons

* qt/pinentrydialog.cpp (PinEntryDialog::setOkText,
PinEntryDialog::setCancelText): Do not set accessible description.

It makes no sense to set the button text also as accessible description.
If screen readers read the description, then they read the same text
twice. Moreover, if the text contains an '&' for marking a keyboard
accelerator, then this '&' is spoken literally at least by NVDA. (Qt
removes the '&' from the text when used as accessible name.)

• GnuPG-bug-id: T5863