Remove frontend

[wolfv]

We've moved the quetz-frontend into a seperate repo: https://github.com/mamba-org/quetz-frontend

[hbcarlos]

Thanks @wolfv. We need to remove it also from here:

quetz/Dockerfile

Lines 4 to 7 in 41a5ec2

	COPY quetz_frontend /quetz_frontend
	RUN cd /quetz_frontend \
	&& npm install \
	&& npm run build

and

quetz/Dockerfile

Line 24 in 41a5ec2

COPY --from=node /quetz_frontend/dist /quetz-frontend

[hbcarlos]

Thanks @wolfv!