docs: remove a dependency on PBR
docs: remove a dependency on PBR

In the past, we had some manual version info in this file, but
apparently it never got updated past the 0.1 release (yay). The, in
4d5d10935 I changed this to retrieve the version information via PBR's
existing magic. That worked well, showing the abbreviated commit hash at
the footer on ReadTheDocs, but I don't think it was terribly useful:

- it was just a hash, nothing like the output of `git describe` (i.e.,
no tag info was there),
- it wasn't showing up in local docs builds which use the Alabaster
default theme.

Now, in 1b2eb9a5 I split the dependency handling. The idea was to make
sure that a "regular installation" would not pull the docs in
needlessly. Unfortunately, the way I set up the RTD builds, the common
dependencies were skipped, and as a result, the `pbr` package was not
available to RTD, and stuff failed.

I could have configured RTD to *also* install from the main deps, but I
don't think this is actually needed. Given that the version info is
rather subtle, let's skip it altogether. Instead, it's better to use
RTD's built-in features for switching between docs for a particular
release (via a git tag) and for the generic "master branch".

Change-Id: Iab0cc9608fa6c24eba93d772370ecd379cf65b1e
Fixes: 1b2eb9a5 (docs: separate out dependencies)

Telecominfraproject/gnpy