Remove "Ubuntu" identifier from User Agent strings

[Wes]

The Chromium and Firefox builds add an extra Ubuntu identifier in their user-agent strings. The inclusion of the OS name is unnecessary and could contribute to user fingerprinting.

Other distros such as Fedora do not add their name to the distro browser builds' UA as far as I can tell, and google-chrome UAs do not reveal OS distro version info.

chromium-browser:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/60.0.3112.113 Chrome/60.0.3112.113 Safari/537.36

Change to:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chromium/60.0.3112.113 Chrome/60.0.3112.113 Safari/537.36

Firefox:
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:55.0) Gecko/20100101 Firefox/55.0

Change to: Mozilla/5.0 (X11; Linux x86_64; rv:55.0) Gecko/20100101 Firefox/55.0

In Firefox the build ID can identify the package build anyway so it isn't necessary to have the distro in the UA string.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Wes (wesinator)]

This was fixed in Firefox 57 but reintroduced in 57.0.1

[Paolo Melchiorre (paulox)]

Please consider removing unnecessary OS information form Chromium.

[Wes (wesinator)]

Also, the chromium snap UA has extra info identifying it as a snap:
"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) snap Chromium/72.0.3626.96 Chrome/72.0.3626.96 Safari/537.36"