refactor(mailjet): remove app region tag from MailjetServlet

[Kef131]

Description

Fixes #
b/347825621

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.
• Tests pass: mvn clean verify required
• Lint passes: mvn -P lint checkstyle:check required
• Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only

Compliance & Style

• I have followed Sample Format Guide
• pom.xml parent set to latest shared-configuration
• Appropriate changes to README are included in PR
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample

Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request removes the // [START app] and // [END app] region tags from the MailjetServlet.java file. There are no review comments, and I have no feedback to provide.