fix: Remove priority from MX record format

- MX records should only contain hostname strings
- Priority is handled by DNS system configuration
- Follow same format as other domains in repository

domains/sam2-saas-prod.is-not-a.dev.json

@@ -12,7 +12,7 @@
       "54.204.75.7"
     ],
     "MX": [
-      "10 feedback-smtp.us-east-1.amazonses.com"
+      "feedback-smtp.us-east-1.amazonses.com"
     ],
     "TXT": [
       "v=spf1 include:amazonses.com ~all",