Remove option to build without Allen

Now that the compile time of Allen has been reduced, the option to build Moore without Allen should be removed.

added Build label

requested review from @rmatev

this needs rebasing.

I prefer to hold off merging this a bit to give time to people to modify their config before the Moore_WITH_Allen cmake cache variable disappears under their feet.

added 81 commits

• f0188f17...bbdac258 - 80 commits from branch master
• c332fab9 - Remove option to build without Allen

Compare with previous version

The rebase is done. Feel free to merge when you think it's the right time.

@raaij @rmatev opinions on this mattermost discussion from yesterday?

https://mattermost.web.cern.ch/lhcb/pl/awp8y3bre7b49xerdpfqsq945w

assigned to @ascarabo

mentioned in issue #351 (closed)

assigned to @rmatev and unassigned @ascarabo

resolved all threads

approved this merge request

added cleanup label

/ci-test --platforms=x86_64_v2-centos7-gcc11-opt

added ci-test-triggered label

• [2021-11-22 12:45] Validation started with lhcb-master-mr#3169

resolved all threads

merged

mentioned in commit 23f804f8

mentioned in issue #356 (closed)