Before v1.12, IRB avoids loading `~/.irbrc` if the project folder has
a `.irbrc` file. So to avoid loading dev's `~/.irbrc`, cfe908c added an
empty irbrc fixture as a workaround.

However, because IRB v1.12 starts loading `~/.irbrc` even if the project
folder has a `.irbrc` file, the workaround no longer works.

This commit removes the workaround and uses altering `HOME` environment
variable to avoid loading `.irbrc` from devs' home directory instead.