Don't create two parse nodes for each => token. (#7894)

Instead of producing two parse nodes in a row for each `=>` token,
produce just one.

Assisted-by: Gemini via Antigravity

toolchain/parse/testdata/lambda/lambda.carbon

@@ -111,7 +111,6 @@ fn G() {
 // CHECK:STDOUT:   │ │   │ ├─LetBindingPattern ':'
 // CHECK:STDOUT:   │ │   ├─ExplicitParamList ')'
 // CHECK:STDOUT:   │ │ ╭─LambdaDefinitionStart '=>'
-// CHECK:STDOUT:   │ │ ├─TerseBodyArrow '=>'
 // CHECK:STDOUT:   │ │ ├─IdentifierNameExpr 'x'
 // CHECK:STDOUT:   │ ├─Lambda 'fn'
 // CHECK:STDOUT:   ├─VariableDecl ';'