Simplify MacroDecl syntax to match the accepted proposals

[DougGregor]

• Explanation: Early macro proposals differentiated syntax between "function-like" and "value-like" macros. However, we settled on always making macros "function-like", so remove the "value-like" syntax, which causes the parser and syntax tree to be more complex than necessary (and not represent the actual language grammar).
• Scope: Parser and syntax tree are affected, but no clients ever used this syntax.
• Risk: Very low; it's eliminating parsing and syntax tree support for an experimental syntax that is no longer in use.
• Reviewer: @ahoppen

[DougGregor]

@swift-ci please test