Remove comma-separated field declaration syntax from this proposal

[littledan]

Comma-separated field declarations were added relatively recently. Let's split out comma-separated field declarations into a separate proposal to be considered with more time to think over some issues.

Comma-separated field declarations have led to a few sources of confusion, including:

• In conjunction with decorators, should the decorator apply to only one of them, or all of them? The intention was to apply to all, but some find it ambiguous.
• Wondering whether we should have trailing commas or some other modified interaction with ASI.
• Additional complexity and general confusion about the motivation.
• There is some implementation burden (e.g., in Babel) for the comma-separated declarations, e.g., to ensure the decorator is evaluated once.

None of these are fatal; they would just be reasonable to spend more time thinking about before including in a Stage 3 proposal. My plan is to back these out for now, and work with @wycats to make it possible to propose this separately.