Remove leftover customization for RTD theme

Now that we use PyData Sphinx Theme, we can get rid of the link
customization meant for the RTD theme. This leftover template
was breaking the build with PyData Sphinx Theme 0.13.0.

docs/_templates/breadcrumbs.html

-{% extends "!breadcrumbs.html" %}
-
-{# Overrides RTD's default "View page source" link formatting, which
-   forces the full page name in the link; but we don't want it because
-   page names include the repo, eg. "swh-model/index".
-   The view_in_gitlab.py extension sets swh_source_url to
-   ".../source/swh-model/browse/master/docs/index"
-#}
-{%- block breadcrumbs_aside %}
-  <li class="wy-breadcrumbs-aside">
-    {%- if hasdoc(pagename) and display_vcs_links %}
-      <a href="{{ swh_source_url }}{{ page_source_suffix }}?plain=1">{{ _('View page source') }}</a>
-    {% endif %}
-  </li>
-{%- endblock %}