Clean out content related to get-app

[maureenlholland]

Description

Follow up from new refresh redirects: https://mozilla-hub.atlassian.net/browse/WT-925
#1216
#1234

Decision: "/mobile/get-app/ can be depreciated as /mobile/ covers this now. No need for email"

The "send to device" email form should be removed as well

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Success Criteria

• get-app template and all corresponding assets/code removed
• send to device email form and all corresponding tests removed

[janbrasna]

Given this was being indexed:

 

I'd also add an item for "redirects to /mobile added".

Especially since e.g. for my geo/locale/OS it's the third result, second on springfield, and the first relevant to my platform:

[janbrasna]

Word of caution when removing the s2d functionality, as there might be forms still hidden here and there. I just found it on https://www.firefox.com/de/download/ios/ but that looks like it might need publishing the correct translation page to override it. Which however lead me to one case, where SEND_TO_DEVICE_LOCALES and enabled_wagtail_langs don't overlap.

And that is https://www.firefox.com/zh-TW/download/ios/ — which won't get a CMS version of its page, as zh-TW is not a CMS locale currently. So this is IIUC now the only page that would still support s2d and be publicly available. Removing the s2d functionality should basically turn this locale to QR-only page as all the others, just noting it for being this unique currently.