Hide "edit on" when the version is a tag

[stsewd]

When the version is a tag, user's can't edit that version, only branches can be edited.

Currently, on the rtd theme we only generate the view link (the text says edit, but it's a view link).

As mention here #1820 (comment), there is some code that was never used https://github.com/rtfd/readthedocs.org/blob/0f5d979c221da7140138359fc06660ebec6fff9b/readthedocs/doc_builder/backends/sphinx.py#L127-L132

As we don't use that on the theme, I think it can be removed (or maybe make it available to the theme?). IMO, we are fine deleting that code, the theme does a good job showing only the view link (users can figure out how to edit on each platform).

Close #1820

[codecov]

Codecov Report

Merging #4851 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4851      +/-   ##
==========================================
+ Coverage   76.41%   76.42%   +<.01%     
==========================================
  Files         158      158              
  Lines        9990     9992       +2     
  Branches     1262     1262              
==========================================
+ Hits         7634     7636       +2     
+ Misses       2016     2015       -1     
- Partials      340      341       +1

Impacted Files	Coverage Δ
readthedocs/builds/models.py	75.8% <100%> (+0.19%)	⬆️

[ericholscher]

We should think more about how we represent this data in the theme, but that's a larger issue. This is a good fix for now.

[humitos]

Nice! This is something that have been bothering me for a while :)