updater: remove check for now unused beta file endings

[tooomm]

Related Ticket(s)

• Fixes Remove former file name pattern from in-client updater code #3082

Short roundup of the initial problem

I went forward with my unanswered assumptions from #3083 as that's how I understand it:

File names are now always:

 Win (64bit) --> filename-win64.exe
 Win (32bit) --> filename-win32.exe
 macOS --> filename.dmg
 Ubuntu --> filename.deb

(in comparison to stable releases, filename part for betas only has a -beta or -beta.X tag attached)

So there is no need to check for x86 and x86_64 any longer!

Actually, is this betaEnd and exeBetaName needed alltogether?

We don't use such a pattern like -x86_64_qt5 for any releases anymore.
Don't we differentiate via the download urls between stable and beta?

What will change with this Pull Request?

• remove unused beta naming mentions

[ctrlaltca]

Looks like this is fine to merge, since the only affected users are those on the "beta" release channel that didn't upgrade since more than one year.