build: remove swift-system dependency

[compnerd]

tools-support-core has dropped this dependency which now allows us to remove the dependency as well.

[compnerd]

@swift-ci please test

[compnerd]

CC: @neonichu

[neonichu]

Not sure we can remove it here since SwiftPM still depends on it and sourcekit-lsp depends on SwiftPM

[compnerd]

Oh, I thought we removed it in SwiftPM as well.

[neonichu]

I had a PR for it, but @tomerd was saying we may need the dependency there at some point, so we shouldn't remove it.

[ahoppen]

Closing because as @neonichu said we still have the SwiftSystem dependency in SwiftPM and thus still need it.