drm/i915/selftests: Drop stale timeline constructor assert

Since we pass around encoded parameters to the kernel context
constructor using the ce->timeline pointer, we can no longer assert that
it should be zero for mock timeline construction.

Fixes: cffef56a ("drm/i915/gt: Support multiple pinned timelines")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200731102206.6793-1-chris@chris-wilson.co.uk