[v10] Remove Flat in favor of actual tonemapping.

[DennisSmolek]

With #3610 and other cleanup I think we can drop the "flat" prop as well.

<Canvas flat > //old
<Canvas tonemapping={THREE.NoToneMapping} > // direct

Yes its more verbose but its intentional and wont be confusing when people try to do:

<Canvas legacy flat tonemapping={THREE.AcesTonemapping} >

Or other nightmares I've seen. Its unclear with those prop tags which has priority and what will actually happen to color. This way its very clear whats doing what