Record of the appropriate SETTE revision for the current NEMO version-4.0 point-release candidate and removal of SETTE tag sette_release-4.0-HEAD

This changeset includes an inactive externals definition in the svn:externals property of ^{/NEMO/releases-4.0-HEAD; if activated, this externals definition would map subdirectory sette to the appropriate SETTE revision (}/utils/CI/sette@12135) for testing this NEMO version-4.0 point-release candidate. It also removes the tag ^{/utils/CI/sette_release-4.0-HEAD for the same SETTE revision.}