Revert "fall back to vanilla if xformers is not available (#51)"

[timudk]

This reverts commit ef520df.

@akx this is a good suggestion but has to be tested further internally.

[akx]

@timudk It works fine on my Mac at least (see #60 for proof), and it will in any case raise a warning - the other option is that SDXL 0.9 simply doesn't work when xformers isn't available.

Also, by having it here in the open-source repo you'll have more people testing it. :)

[timudk]

@akx those are good suggestions but we would like to test them internally first.

[akx]

Well, testing won't probably happen if the code isn't in the repo 😁

Again, this only makes things actually work when xformers isn't installed.