drm/ast: Remove AST_TX_ITE66121 constant

The ITE66121 is an HDMI transmitter chip. There's no code for
detecting or programming the chip within ast. Remove the enum
constant.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207141544.30015-4-tzimmermann@suse.de