[PackageGraph] Remove a unnecessary assertion
[PackageGraph] Remove a unnecessary assertion

<rdar://problem/40617721> [SR-7796]: Assertion failed: : file /Users/buildnode/jenkins/workspace/oss-swift-4.2-package-osx/swiftpm/Sources/PackageGraph/PackageGraphRoot.swift, line 124

I was overly strict and added an assertion to check that revision
dependencies have git hash length of 40. I don't think that check is
actually needed because we do validate the git ref.

abertelrud/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,sschiau/swift-package-manager