Xcode: Revert addition of "outputPaths" to custom command build phase
The change in !1054 (merged) 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 (closed)