Fix the hoverxref configuration

[Gallaecio]

You shouldn't override hoverxref_version and hoverxref_project since they are taken automatically from Read the Docs.

If you want to avoid your CI failing because of this, you can define the environment variables as Read the Docs does:

READTHEDOCS_PROJECT=scrapy
READTHEDOCS_VERSION=''

With the current configuration, all the versions built on Read the Docs will point to a different version on Read the Docs and this will conflict. For example, current master version in Read the Docs defines hoverxref_version='2.0.0' but that version does not exist on Read the Docs and the tooltip does not known where to get the content from.

@humitos at #4480 (comment)