Remove HTTP User-Agent farbling

[pilgrim-brave]

Brave's initial implementation of farbling the User-Agent string sent with HTTP requests has proven to be difficult to maintain (subtle changes upstream have lead to regressions that are difficult to debug). With C90's effective freezing of the Mac OS X version string (to 10.5.7, for compatibility reasons), our implementation will provide ever smaller privacy benefits over time.

[stephendonner]

@pilgrim-brave would you please drop a test plan we can follow over in brave/brave-core#8562? Thanks! /cc @pes10k since I see you were also working on abating this with Cloudflare.

Also looping in @brave/legacy_qa

[kjozwiak]

Talked to @pes10k earlier via the uplift thread. Labeling this as QA/No as the checks will be done via #15039, #15322 & #14740. @pes10k let me know if that sounds right or if there's anything else that we need to check for this one.