iris: Drop PIPE_CAP_BUFFER_SAMPLER_VIEW_RGBA_ONLY

iris: Drop PIPE_CAP_BUFFER_SAMPLER_VIEW_RGBA_ONLY

This cap is mainly for working around a r600 texture swizzle issue,
but it also controls whether ARB_texture_buffer_object (with legacy
formats) is enabled.  I suspect the missing I/L/A/LA faking is why
I had it set in the first place.

Thanks to Ilia for pointing out that I shouldn't be setting this.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>