Remove default value for vcs_pageview_mode (#617)

This variable is used on the template and evaluated to
give a default value.

According to http://jinja.pocoo.org/docs/2.10/templates/#default
default only works if the variable is undefined,
or passing true as second argument to evaluate to false.

sphinx_rtd_theme/theme.conf

@@ -15,4 +15,3 @@ logo_only =
 display_version = True
 prev_next_buttons_location = bottom
 style_external_links = False
-vcs_pageview_mode =