Editorial: Merge the RHSs of AsyncFunctionExpression

The nonterminals:
- FunctionExpression
- ClassExpression
- GeneratorExpression
- AsyncGeneratorExpression

are all defined with a single RHS involving an optional BindingIdentifier.

But AsyncFunctionExpression is defined with two RHSs,
one with a BindingIdentifier and one without.
(It's been that way since it was introduced in PR tc39#692.)
I can't see any reason for it to be not like the others.

spec.html

@@ -21236,8 +21236,7 @@ <h2>Syntax</h2>
         [+Default] `async` [no LineTerminator here] `function` `(` FormalParameters[~Yield, +Await] `)` `{` AsyncFunctionBody `}`
 
       AsyncFunctionExpression :
-        `async` [no LineTerminator here] `function` `(` FormalParameters[~Yield, +Await] `)` `{` AsyncFunctionBody `}`
-        `async` [no LineTerminator here] `function` BindingIdentifier[~Yield, +Await] `(` FormalParameters[~Yield, +Await] `)` `{` AsyncFunctionBody `}`
+        `async` [no LineTerminator here] `function` BindingIdentifier[~Yield, +Await]? `(` FormalParameters[~Yield, +Await] `)` `{` AsyncFunctionBody `}`
 
       AsyncMethod[Yield, Await] :
         `async` [no LineTerminator here] PropertyName[?Yield, ?Await] `(` UniqueFormalParameters[~Yield, +Await] `)` `{` AsyncFunctionBody `}`
@@ -21282,9 +21281,7 @@ <h1>Static Semantics: Early Errors</h1>
 
         AsyncFunctionDeclaration : `async` `function` `(` FormalParameters `)` `{` AsyncFunctionBody `}`
 
-        AsyncFunctionExpression : `async` `function` `(` FormalParameters `)` `{` AsyncFunctionBody `}`
-
-        AsyncFunctionExpression : `async` `function` BindingIdentifier `(` FormalParameters `)` `{` AsyncFunctionBody `}`
+        AsyncFunctionExpression : `async` `function` BindingIdentifier? `(` FormalParameters `)` `{` AsyncFunctionBody `}`
       </emu-grammar>
       <ul>
         <li>It is a Syntax Error if ContainsUseStrict of |AsyncFunctionBody| is *true* and IsSimpleParameterList of |FormalParameters| is *false*.</li>
@@ -21335,9 +21332,7 @@ <h1>Static Semantics: Contains</h1>
 
         AsyncFunctionDeclaration : `async` `function` `(` FormalParameters `)` `{` AsyncFunctionBody `}`
 
-        AsyncFunctionExpression : `async` `function` `(` FormalParameters `)` `{` AsyncFunctionBody `}`
-
-        AsyncFunctionExpression : `async` `function` BindingIdentifier `(` FormalParameters `)` `{` AsyncFunctionBody `}`
+        AsyncFunctionExpression : `async` `function` BindingIdentifier? `(` FormalParameters `)` `{` AsyncFunctionBody `}`
       </emu-grammar>
       <emu-alg>
         1. Return *false*.
@@ -21389,9 +21384,7 @@ <h1>Static Semantics: IsConstantDeclaration</h1>
     <emu-clause id="sec-async-function-definitions-static-semantics-IsFunctionDefinition">
       <h1>Static Semantics: IsFunctionDefinition</h1>
       <emu-grammar>
-        AsyncFunctionExpression : `async` `function` `(` FormalParameters `)` `{` AsyncFunctionBody `}`
-
-        AsyncFunctionExpression : `async` `function` BindingIdentifier `(` FormalParameters `)` `{` AsyncFunctionBody `}`
+        AsyncFunctionExpression : `async` `function` BindingIdentifier? `(` FormalParameters `)` `{` AsyncFunctionBody `}`
       </emu-grammar>
       <emu-alg>
         1. Return *true*.