Remove `allow_multiple_sequences` as a concept

[graeme-winter]

Silver-Surfer command_line :) [master] $ git commit -m "Multiple sequences are almost certainly fine; but leave option"
black....................................................................Passed
Check python ast.........................................................Passed
Flake8...................................................................Passed
Check for merge conflicts................................................Passed
Check for added large files..............................................Passed
[master 736cc19c8] Multiple sequences are almost certainly fine; but leave option
 1 file changed, 1 insertion(+), 1 deletion(-)
Silver-Surfer command_line :) [master+1] $ git push
Enumerating objects: 7, done.
Counting objects: 100% (7/7), done.
Delta compression using up to 8 threads
Compressing objects: 100% (4/4), done.
Writing objects: 100% (4/4), 409 bytes | 409.00 KiB/s, done.
Total 4 (delta 3), reused 0 (delta 0)
remote: Resolving deltas: 100% (3/3), completed with 3 local objects.
To github.com:dials/dials.git
   6874e2fc0..736cc19c8  master -> master

main issue will be expunging this from the documentation

[dagewa]

Is this all done? Can be closed?

[graeme-winter]

Yes, yes, well reminded thank you

[Anthchirp]

Would have been better to link the commit (736cc19c8) instead of copy&pasting the git command output :)

In any case this is still referred to in the documentation. Should that be removed?

[graeme-winter]

Yes, fair enough on the commit

Was removing in documentation / small molecule tutorial but need to finish 😕 my bad

[graeme-winter]

Can leave in docs because you can still set to False, but need to fix tutorial