staging: gdm724x: Remove unused driver

staging: gdm724x: Remove unused driver

Won Kang from gct contributed the driver in 2013.

The following reasons lead to the removal:
- This driver generates maintenance workload
- The manufacturer is not interested and does not care as Emails or
  inquiries, to support or involved persons of gct, got unanswered.
- Did not find a possibility to buy the chips.
- Did not find minimal documentation on the web.
- Did not find a device where it is build in and the user is able to
  install any Linux. Therefore it is not possible to do any testing of
  the driver from the community.
- No blog entries about anyone using the gdmtty and gdmulte.
- No response about usage of this drivers to the Email from April 2024

Link: https://lore.kernel.org/linux-staging/2024100910-smoky-condiment-2298@gregkh/T/#u
Link: https://lore.kernel.org/all/78b521eb-4e89-4c01-8dfc-1fb990e6887d@gmail.com/
Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/20241010191508.21055-1-philipp.g.hortmann@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>