webui.sh: remove all `cd` related code

This may be helpful for
https://github.com/AUTOMATIC1111/stable-diffusion-webui/issues/7028,
because we won't change working directory to the repo now, instead,
we will use any working directory. If we set working directory to
a path contains repo and the custom --data-dir, the problem in this
issue should be solved.

Howewer, this may be treated as an incompatible change if some code
assume the working directory is always the repo.

Also, there may be another solution that always let --data-dir be the
subdirectory of the repo, but personally I think this may not be what
we actually need.

As this issue mainly influent on Docker and I am not familiar with
.bat files, updating webui.bat is skipped.

webui.sh: source env from repo instead $PWD