telegram-desktop shouldn't have forced gtk dialog

telegram-desktop package uses -DTDESKTOP_FORCE_GTK_FILE_DIALOG=ON what prevents using native dialogs via platformtheme plugins (e.g. kde dialog).

Also, it have https://git.alpinelinux.org/aports/tree/community/telegram-desktop/qt-plugin-path.patch#n13 that sets QT_PLUGIN_PATH and unsets various scale variables, but:

1. telegram-desktop works fine without QT_PLUGIN_PATH since migrated to cmake
2. scale-related variables already managed in Ui::DisableCustomScaling()
3. After removing -DTDESKTOP_FORCE_GTK_FILE_DIALOG=ON, there is no need to unset QT_QPA_PLATFORMTHEME and QT_STYLE_OVERRIDE

I.e. the patch can be completely dropped