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