fix: remove x-sourcemap header

This header is useless, as the client bundle already contains a
sourceMappingURL field.

Besides, Firefox prints the following warning:

> <url> is being assigned a //# sourceMappingURL, but already has one

Related: #3958

Author: darrachequesne

lib/index.ts

@@ -515,9 +515,6 @@ export class Server<
     );
     res.setHeader("ETag", expectedEtag);
 
-    if (!isMap) {
-      res.setHeader("X-SourceMap", filename.substring(1) + ".map");
-    }
     Server.sendFile(filename, req, res);
   }
 

test/socket.io.ts

@@ -59,7 +59,7 @@ describe("socket.io", () => {
             if (err) return done(err);
             expect(res.headers["content-type"]).to.be("application/javascript");
             expect(res.headers.etag).to.be('"' + clientVersion + '"');
-            expect(res.headers["x-sourcemap"]).to.be(filename + ".map");
+            expect(res.headers["x-sourcemap"]).to.be(undefined);
             expect(res.text).to.match(/engine\.io/);
             expect(res.status).to.be(200);
             done();