Remove redundant StringMap in protocol tests

smithy-aws-protocol-tests/model/restJson1/http-prefix-headers.smithy

@@ -8,6 +8,7 @@ namespace aws.protocoltests.restjson
 use aws.protocols#restJson1
 use smithy.test#httpRequestTests
 use smithy.test#httpResponseTests
+use aws.protocoltests.shared#StringMap
 
 /// This examples adds headers to the input of a request and response by prefix.
 @readonly
@@ -86,11 +87,6 @@ structure HttpPrefixHeadersInputOutput {
     fooMap: StringMap,
 }
 
-map StringMap {
-    key: String,
-    value: String,
-}
-
 /// Clients that perform this test extract all headers from the response.
 @readonly
 @http(uri: "/HttpPrefixHeadersResponse", method: "GET")