Revert "ExternalData: determine the data path properly"

This reverts commit 8a3c081b94ea72bef9a7afb58937e9a4f1fa4b5c.
The case where this fails will have other problems outside
of ExternalData.  We need to fix this in CheckTranslationPath
instead.
Merge topic 'dev/fix-externaldata-with-symlink-in-source-path' into next

91717fa5 Revert "ExternalData: determine the data path properly"