Remove redundant usage of Gitlab::I18n.with_locale in mailers

What does this MR do and why?

This commit removes the redundant usage of Gitlab::I18n.with_locale in mailer methods that already use mail_with_locale or email_with_layout.

These methods already internally set the locale based on the user's preferred language, so wrapping them with Gitlab::I18n.with_locale is unnecessary.

Issue: #417236 (closed)

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