Stop handling package issues from 'sphinx-build'

There were a number of package error handlers run as part of the
'sphinx-build' command/executable:

- Unsupported Python version (it should be 2.7 or 3.4+)
- Missing packages (missing docutils, jinja2, and roman, which is part
  of docutils, packages)
- Out-of-date packages (docutils)

This code is mostly unchanged since Sphinx was first released. Python,
and in particular Python's packaging options, have come a long way since
then. Today, all of the above checks are provided by setuptools and the
'setup.py' script, meaning we should never actually get to the point of
triggering any of these checks. This is further reinforced by the fact
that none of the other executables carry out these checks: either this
is a bug that no one has reported in ~8 years or, more likely, the
checks are useless and we don't need them anywhere.

In all, we can happily remove these checks, greatly simplify a piece of
code that's otherwise rarely touched, and trust that setuptools is up to
the job it's designed for.

Signed-off-by: Stephen Finucane <stephen@that.guru>

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