Revert "Help: Update cmake-packages.7 examples to import targets only once"

This reverts commit cfab942e5ff8463f8053564e68179da110280b43.
Since commit v2.8.11~362^2 (Generate an early-return guard in target
Export files, 2012-11-30) an external include guard is not needed.
Merge topic 'doc-package-target-import-once' into next

0fee3f47 Revert "Help: Update cmake-packages.7 examples to import targets only once"