Add comments about Gamecore Scalarization and remove old confusing behaviour about SSR feature flag

[FrancescoC-unity]

Pretty much all in title, all trivial things at the end.

• Gamecore situation: it is in the plan to be looked at, but no ETA, so I added a comment.
• The LIGHTFEATUREFLAGS_SSREFLECTION situation is just adding confusion at the moment and has no use currently.

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