Xcode: Revert addition of "outputPaths" to custom command build phase

The change in commit v3.9.0~3^2 (Xcode: Add "outputPaths" to custom
command script build phase, 2017-07-13) was meant to support Xcode 9's
new build system.  However, without matching "inputPaths", Xcode will
not re-run the build phase if its outputs have already been generated.
This broke the old Xcode build system too.

Revert the change for now so at least the old Xcode build system works.
Further investigation will be needed to add proper support for Xcode 9's
new build system.

Fixes: #17178

Help/release/3.9.rst

@@ -34,8 +34,6 @@ Generators
   This is an experimental feature and can be activated by setting the
   :variable:`CMAKE_XCODE_GENERATE_SCHEME` variable to a ``TRUE`` value.
 
-* The :generator:`Xcode` generator now supports Xcode 9.
-
 Commands
 --------
 
@@ -337,3 +335,8 @@ Changes made since CMake 3.9.0 include the following.
 * On macOS, the default application bundle ``Info.plist`` file no longer
   enables Hi-DPI support as it did in 3.9.0 and 3.9.1.  The change had
   to be reverted because it broke iOS applications.
+
+* The Xcode generator no longer adds "outputPaths" to custom script
+  build phases as it did in 3.9.0 and 3.9.1.  This was added in an
+  attempt to support Xcode 9's new build system, but broke incremental
+  rebuilds for both the old and new Xcode build systems.

Source/cmGlobalXCodeGenerator.cxx

@@ -1517,17 +1517,6 @@ void cmGlobalXCodeGenerator::AddCommandsToBuildPhase(
   makecmd += " all";
   buildphase->AddAttribute("shellScript", this->CreateString(makecmd));
   buildphase->AddAttribute("showEnvVarsInLog", this->CreateString("0"));
-
-  cmXCodeObject* outputFiles = this->CreateObject(cmXCodeObject::OBJECT_LIST);
-  for (std::vector<cmCustomCommand>::const_iterator i = commands.begin();
-       i != commands.end(); ++i) {
-    std::vector<std::string> const& outputs = i->GetOutputs();
-    for (std::vector<std::string>::const_iterator j = outputs.begin();
-         j != outputs.end(); ++j) {
-      outputFiles->AddObject(this->CreateString(*j));
-    }
-  }
-  buildphase->AddAttribute("outputPaths", outputFiles);
 }
 
 void cmGlobalXCodeGenerator::CreateCustomRulesMakefile(