doc: Remove sphinxcontrib-seqdiag Python package from RTD builds

doc: Remove sphinxcontrib-seqdiag Python package from RTD builds

This is a proactive PR to avoid breaking docs builds when Setuptools 81
starts to be used in the RTD builds process.

The sphnixcontrib-seqdiag Python package is not compatible with
Setuptools 81 or later due to use of pkg_resources:
https://setuptools.pypa.io/en/latest/pkg_resources.html

Setuptools 81 release should be imminent, with the Python deprecation
warning stating pkg_resources "removal as early as 2025-11-30".

Seqdiag seems to be unmaintained with the latest update at Pypi in
the year 2021 and also no updates to the seqdiag git repo.

There are no seqdiag directives left in the docs after last seqdiags
were removed in PR #52308.

Two other options would exist for fixing the situation (see PR for
discussion) but this seems to be the suitable one.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>