[Fix] Fix in lightmapper where a material lightmap would be incorrectly released during baking

[mvaligursky]

• lightmapper was assigning a material specified lightmap to a baked lightmap slot on the mesh instance.
• when this was followed by lightmap baking, a baked lightmap would be assigned to that spot, and the ref counted original texture would be destroyed
• I do not see a reason for assignment at all, as we're baking the texture, and so I removed it.
• avoids crash in Editor when an asset lightmap texture was destroyed .. and any operation on (toggling mimaps) would cause a null access on destroyed texture