Remove performance.html references

[reemhamz]

One-line summary

Removed references to the now obsolete performance.html file

Significant changes and points to review

A string update was requested for a Firefox Developer page, which was done here: #14021, however since references to the for-developer.html template (that had the updated string) were being conditionally added to several files with the ftl_has_messages helper with the conditions of the now-obsolete strings being available, I removed the ftl helper and also removed the older performance.html template that was under the else option, which was isn't even being rendered anymore because the conditional logic was placed years ago, and relevant strings had already been translated.

Issue / Bugzilla link

This is an extra ticket to finish off #13970

Testing

• http://localhost:8000/en-US/firefox/developer/
• http://localhost:8000/en-US/firefox/99.0a2/whatsnew/
• http://localhost:8000/en-US/firefox/99.0a2/firstrun/

[stephaniehobson]

R+ ⏩