Revert "Merge pull request #145 from rintaro/typealias-as-associatedtype"
Revert "Merge pull request #145 from rintaro/typealias-as-associatedtype"

This reverts commit 5bfc40ba33e8b6555336bbf691aeda9621806f27, reversing
changes made to adc26e4473d53e6b3e1b7945aca495595f07cc41.

apple/swift-syntax,apple/swift-syntax,apple/swift-syntax