staging: rtl8723bs: remove IsHexDigit

Remove function IsHexDigit because it is not used.

Signed-off-by: Nam Cao <namcaov@gmail.com>
Link: https://lore.kernel.org/r/8c70ecd9bea1dff2cb8f69e12bbef6aa4ee39977.1662111799.git.namcaov@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>