Revert all lightgrid changes introduced in Alpha 25. This is a combination of the following three commits:

Revert "Let r_AmbientScale scale the outgoing diffuse light."
This reverts commit 2f89a596972d1367e345f714f3e4f15573b85d8b.

Revert "Reactivate r_ambientScale and r_normalScale cvars."
This reverts commit 0a13b9a7fe588b36256b42abb679ba3c1ae63af7.

Revert "Revert "Revert all the vertex pack specific commits (lightgrid changes)""
This reverts commit f9adfe040db42edb2f8e2e21f1918ca429249a8f.