v3dv: drop incorrect assertion

It is possible to specify a depth/stencil clear but then have no
actual depth/stencil attachment used in the subpass. In that case
we are already skipping the store.

Fixes:
dEQP-VK.renderpass.suballocation.unused_clear_attachments.*depthonly_unused

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>
1 file changed