Use MultiLinearGeometry to implement UGGrid local geometries, rathern than CachedMultiLinearGeometry

Caching was never the reason why CachedMultiLinearGeometry was used here.
Rather, the MultiLinearGeometry class contained a bug which made it impossible
to evaluate some of the Geometry methods at simplex corners.  This bug
was fixed by Ansgar Burchardt in dune-geometry commit 7ade11ecbfbbe327b5ef647f10e2f5c8007b38ab
We can therefore safely switch to the simpler MultiLinearGeometry code now.

Incidentally, this resolves the thread-safety worries that Jö expressed in
the second document attached to FS 1717.