tty: hvc: remove hvcs_driver_string

tty: hvc: remove hvcs_driver_string

No users of hvcs_driver_string, remove it. This fixes the following gcc
warning:

drivers/tty/hvc/hvcs.c:199:19: warning: ‘hvcs_driver_string’ defined but
not used [-Wunused-const-variable=]
 static const char hvcs_driver_string[]
                   ^~~~~~~~~~~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200403071325.3721-1-yanaijie@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>