In Cycles the volume transmittance is already composited into the color
passes. In Eevee the volume transmittance pass was separate and needed
to be composited in the compositor. This patch adds the volume
transmittance pass direct in the next render passes:

• Diffuse Color
• Specular Color
• Emission
• Environment

This patch includes the removal of the volume transmittance render pass.
It also renames the volume render passes to match Cycles. The setting
themselves aren't unified.