Xcode: Drop check for circular target dependencies

The modern cmComputeTargetDepends guarantees there will by no cycles.
Drop the Xcode generator's check which only handled length 1 and 2
cycles anyway.
Xcode: Each target dependency edge needs a unique object (#13935)

Generate a unique PBXTargetDependency and PBXContainerItemProxy for
every edge in the dependency graph as required by the Xcode format.
Xcode only accidentally loads project files that re-use the same
PBXTargetDependency for every edge leading to a single target.  If one
is removed by hand in the IDE then Xcode may crash due to dangling
references from the others.

Since cmGlobalXCodeGenerator::AddDependTarget is called exactly once for
every edge in the target dependency graph we do not need to keep track
of which edges have been visited to avoid generating duplicates.
Merge topic 'xcode-target-depends' into next

b005140 Xcode: Each target dependency edge needs a unique object (#13935)
781ea6d Xcode: Drop check for circular target dependencies