v3dv: remove unused clamp_to_transparent_black_border property

v3dv: remove unused clamp_to_transparent_black_border property

Commit e07c5467 ("v3dv/format: use XYZ1 swizzle for three-component formats")
removes the only code that handled the clamp_to_transparent_black_border
variable. Therefore, the variable can be deleted, as it is not currently
being used.

Fixes: e07c5467 ("v3dv/format: use XYZ1 swizzle for three-component formats")
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20746>
(cherry picked from commit 86c9bdcd9adc952654f58329d4d482b7a509f5c7)