plugins/sourcekit-lsp: Don't register formatter for Swift

sourcekit-lsp does not support formatting Swift

For formatting, https://github.com/apple/swift-format could be used. I have a plugin for that - already in C - if wished.

Ref: https://github.com/apple/sourcekit-lsp/issues/576 Ref: https://github.com/apple/sourcekit-lsp/pull/361