Deprecate SecureDropUpdater page

[deeplow]

Describe the change

updatergui_development.rst no longer makes sense to exist:

• we're no longer using .qrc files
• Perhaps the relevant part about .ui files could be moved to sdw-launcher/ (under https://github.com/freedomofpress/securedrop-workstation)

How will this impact users?

No impact

User Research Evidence

None