[create] Remove --no-git flag

[fabianegli]

Description of feature

As discussed in Slack (this and following posts) the option to create workflows without using git will be removed if no compelling use cases come to our attention.

Quote from @ewels about how the git-less creation came to be:

I think it was requested for people who had already initialised a git repo themselves previously
But it's generally A Bad Thing, as the fresh repo is needed for automated template synchronisation

Context: #2074 (comment)

[awgymer]

I was just reading through the docs for nf-core sync and under the section on syncing retrospectively I noticed the following:

If your pipeline already has versioned releases (eg. you are not currently on 1.0dev), then specify the version number that you are currently on:
nf-core create --no-git --version 1.3dev

So I am wondering whether there may be a legitimate edge-case where we need to run create without actually initialising any git repo.

[ewels]

Yup, but syncing retrospectively is an utterly horrible experience that should be avoided at all costs 😆 It was more uncommon in the early days of nf-core when were first setting up the template sync mechanism, but in recent years I think that almost no-one has needed to do it.

Removing the --no-git option is one way of discouraging this. And doing the equivalent manually (eg. copying the files out and over) is not particularly difficult.