Blonk out text version of offline event template

Blonk out text version of offline event template
As our sendMessage protocol generates text versions at runtime
and the audience for these is vanishingly small we are moving towards
removing the shipped text versions of workflow messages in favour
of runtime-generated. The plan is to do this
with the offline-participant-receipt template first. By blanking it
out people who have not customised it should get the blank
version on upgrade.

Merging this part does not require any additional action but there
are some follow ons