remove mention of string

Author: yoshuawuyts

design/mvp/WIT.md

@@ -988,12 +988,8 @@ deprecated-gate ::= '@deprecated' '(' version-field ')'
 
 feature-field ::= 'feature' '=' id
 version-field ::= 'version' '=' <valid semver>
-message-field ::= 'message' '=' <core:string>
 ```
 
-In this syntax `<core:string>` refers to core Wasm's [string
-value](https://webassembly.github.io/spec/core/text/values.html#text-string).
-
 As part of WIT validation, any item that refers to another gated item must also
 be compatibly gated. For example, this is an error:
 ```wit