xen kconfig: don't "select INPUT_XEN_KBDDEV_FRONTEND"

The Xen framebuffer driver selects the xen keyboard driver, so the latter
will be built-in if XEN_FBDEV_FRONTEND=y. However, when CONFIG_INPUT
is a loadable module, this configuration cannot work. On mainline kernels,
the symbol will be enabled but not used, while in combination with
a patch I have to detect such useless configurations, we get the
expected link failure:

drivers/input/built-in.o: In function `xenkbd_remove':
xen-kbdfront.c:(.text+0x2f0): undefined reference to `input_unregister_device'
xen-kbdfront.c:(.text+0x30e): undefined reference to `input_unregister_device'

This removes the extra "select", as it just causes more trouble than
it helps. In theory, some defconfig file might break if it has
XEN_FBDEV_FRONTEND in it but not INPUT_XEN_KBDDEV_FRONTEND. The Kconfig
fragment we ship in the kernel (kernel/configs/xen.config) however
already enables both, and anyone using an old .config file would
keep having both enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: David Vrabel <david.vrabel@citrix.com>
Fixes: 36c1132e ("xen kconfig: fix select INPUT_XEN_KBDDEV_FRONTEND")
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>