gallium: Remove `PIPE_CAP_RGB_OVERRIDE_DST_ALPHA_BLEND`

gallium: Remove `PIPE_CAP_RGB_OVERRIDE_DST_ALPHA_BLEND`

Since the mesa state tracker can promote RGB texture formats
to RGBA texture formats (among other formats) without exposing
any of that information to a driver, it is more desirable to
have the behaviour of `PIPE_CAP_RGB_OVERRIDE_DST_ALPHA_BLEND`
be the default. This avoids rendering bugs where an application
sets `DST_ALPHA` blending on a format where there is no alpha
channel, that has been promoted to a format that actually has an
alpha channel. The driver can instead rely on the common code
in the state tracker to convert the blending parameter to one
that reflects the limitations of the application requested format,
as long as `PIPE_CAP_INDEP_BLEND_FUNC` is supported.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24044>