[lldb] Remove Xcode project legacy

Summary: Since D65109 removed the manually maintained Xcode project, there's a few things we don't need anymore. Anything here we should keep or anything more to remove?

Reviewers: JDevlieghere, jasonmolenda, clayborg, jingham, lanza, teemperor

Subscribers: mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65155

llvm-svn: 366879