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
9 jobs for revert-xcode-9-new-buildsystem-support
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