cleanup: Simplify usage of MinIOSourceProxyRequest (#19553)

This replaces a convoluted condition that ultimately evaluated to

"is this HTTP header present in the request or not?"

cmd/object-api-options.go

@@ -66,13 +66,8 @@ func getDefaultOpts(header http.Header, copySource bool, metadata map[string]str
 	if crypto.S3.IsRequested(header) || (metadata != nil && crypto.S3.IsEncrypted(metadata)) {
 		opts.ServerSideEncryption = encrypt.NewSSE()
 	}
-	if v, ok := header[xhttp.MinIOSourceProxyRequest]; ok {
-		opts.ProxyHeaderSet = true
-		opts.ProxyRequest = strings.Join(v, "") == "true"
-	}
-	if _, ok := header[xhttp.MinIOSourceReplicationRequest]; ok {
-		opts.ReplicationRequest = true
-	}
+	_, opts.ProxyRequest = header[xhttp.MinIOSourceProxyRequest]
+	_, opts.ReplicationRequest = header[xhttp.MinIOSourceReplicationRequest]
 	opts.Speedtest = header.Get(globalObjectPerfUserMetadata) != ""
 	return
 }