s3api: preserve Host header port in signature verification (#8434)

Avoid stripping default ports (80/443) from the Host header in extractHostHeader.
This fixes SignatureDoesNotMatch errors when SeaweedFS is accessed via a proxy
(like Kong Ingress) that explicitly includes the port in the Host header or
X-Forwarded-Host, which S3 clients sign.

Also cleaned up unused variables and logic after refactoring.

weed/s3api/auto_signature_v4_test.go

@@ -415,13 +415,13 @@ func TestSignatureV4WithoutProxy(t *testing.T) {
 			name:         "HTTP with standard port",
 			host:         "backend:80",
 			proto:        "http",
-			expectedHost: "backend",
+			expectedHost: "backend:80",
 		},
 		{
 			name:         "HTTPS with standard port",
 			host:         "backend:443",
 			proto:        "https",
-			expectedHost: "backend",
+			expectedHost: "backend:443",
 		},
 		{
 			name:         "HTTP without port",
@@ -451,13 +451,13 @@ func TestSignatureV4WithoutProxy(t *testing.T) {
 			name:         "IPv6 HTTP with standard port",
 			host:         "[::1]:80",
 			proto:        "http",
-			expectedHost: "::1",
+			expectedHost: "[::1]:80",
 		},
 		{
 			name:         "IPv6 HTTPS with standard port",
 			host:         "[::1]:443",
 			proto:        "https",
-			expectedHost: "::1",
+			expectedHost: "[::1]:443",
 		},
 		{
 			name:         "IPv6 HTTP without port",