Fix warnings part 1

[Saibamen]

Remove unuses usings. Remove redundant (). Fix one CS4014. Add missing braces for foreach. Remove redundant else. Fix other warnings

[Saibamen]

@marticliment Can you please review this? I'm still fixing merge conflicts.

[marticliment]

Hey @Saibamen,

I really appreciate the effort put into improving the codebase, but please do not push PRs with style changes (e.g. replacing new() with [], adding/removing optional brackets, spaces after the if clause, etc.), since they are very time-consuming to review, and don't offer a real benefit on code performance or efficiency.
However, the efficiency changes (IsEmpty, Count == 0) are great.

Thanks for the effort!
Martí