drm/i915: Remove prototype for intel_cx0_phy_ddi_vswing_sequence()

That function is not defined anywhere.

Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Ravi Kumar Vodapalli <ravi.kumar.vodapalli@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613214658.1099759-1-gustavo.sousa@intel.com