build: remove SwiftSystem check

The swift-system package is not used in SourceKit LSP. It was required
to fulfil the dependencies for swift-package-manager. With the changes
in swiftlang/swift-package-manager#8231 which internalised the
SystemPackage import, we no longer need this to be provided.

Author: compnerd

CMakeLists.txt

@@ -23,7 +23,6 @@ find_package(SwiftPM QUIET)
 find_package(LLBuild QUIET)
 find_package(ArgumentParser CONFIG REQUIRED)
 find_package(SwiftCollections QUIET)
-find_package(SwiftSystem CONFIG REQUIRED)
 find_package(SwiftSyntax CONFIG REQUIRED)
 find_package(SwiftCrypto CONFIG REQUIRED)