Further RM biome tweaks

• Make Savanna and Nubia have a .25 Forest probability.
• Remove brightness changes from HDR, this made postproc look too different, particularly on Savanna
• Lower postproc impact on constrast/saturation for the same reasons.
• Lighten up Aegean, Alpine, Savanna, Nubia, slightly darken Arctic
• Make Aegean water a bit bluer. Likewise lightwen Savanna water.
• Fix the sun elevation/rotation on Nubia, Sahara and Savanna
• Remove the HDR settings of Cantabrian Highlands
• Remove the water/HDR settings of Corinthian Isthmus RM

Comments by: marder

Differential Revision: https://code.wildfiregames.com/D4124