braille: remove fallback on VT 7
X or Wayland is less and less actually running on VT 7. libbrlapi has been enhanced to ask logind which VT we are running on even when WINDOWPATH or XDG_VTNR is not set, so avoid interfering with that by passing a hardcoded VT number.
At worse, users can use XDG_VTNR to override the VT number determined by libbrlapi.