[PM-30613] Remove unused mjml-based-email-templates feature flag

[trmartin4]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-30613

📔 Objective

The mjml-based-email-templates flag was used to gate which method we used to send the Send OTP emails, as when we developed it we weren't sure whether MJML email services were going to be ready.

Since they are ready for use, and the Send OTP email work is still in development to be consumed by the Tools team, adding this feature flag was not necessary and adds additional complexity and permutations to their testing. Rather than having Tools test with the flag off then the flag on, we'll just have it on to begin with and they'll use the new email as the basis for their testing.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[github-actions]

Checkmarx One – Scan Summary & Details – acf57895-0d3c-4ba2-b6f5-194e03063ebb

Great job! No new security vulnerabilities introduced in this pull request

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.05%. Comparing base (80d05ee) to head (5a60e4b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6889      +/-   ##
==========================================
- Coverage   56.07%   56.05%   -0.02%     
==========================================
  Files        1968     1968              
  Lines       86927    86897      -30     
  Branches     7742     7741       -1     
==========================================
- Hits        48740    48711      -29     
  Misses      36386    36386              
+ Partials     1801     1800       -1     

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