Slightly cleanup DLMAN

[nico-abram]

commit messages have the description of the stuff I did. Aggregated:

• Removed unused ReadThis class and functions
• removed unused cpuid in ComputerIdentity()
• Inlined progressfunc and write_data definitions to the only place they're used as capture-less lambas
• Inlined checkProtocol function which had a confusing name and was only used once (Also changed default to https)
• Removed the hacky EncodeSpaces function, replaced it with curl's url encoding function (curl_easy_escape) in the places where I think it was needed (tested score leaderboard requests and pack downloading and they seem to work alright)
• Remove ends_with function from DLMAN (Already in RageUtil.h)
• Remove unused addFileToForm function from DLMAN (unused)
• Remove SetCURLPostToURL which was only called in 2 places and only added one line over SetCURLURL
• Remove an unused CURL handle fromDownloadManager::RefreshTop25 which I think we were leaking.
• Move CURL handle deletion of the Download struct from ~DownloadManager to it's destructor ~Download
• Removed the unused Done() callback from Download
• Remove split (unused, probably replaced with splitPath at some point and not removed)
• Renive findStringIC (unused)
• Remove duplicated GetChartLeaderboard AddMethod call
• Replace a couple 3-line empty lambda definitions which just add noise with {} literals which are equivalent
• Remove unused CURLMsg* param from DLMAN callbacks

[nico-abram]

Note: The alias I removed had a lowercase b instead and is used in 'til death, so I added it back

[nico-abram]

Superceded by https://github.com/etternagame/etterna/pull/1200