Remove the sphinxcontrib-seqdiag dependency as the Pillow upgrade to
version 10.x (from OpenStack upper constraints) breaks its usage.
In the ironic source docs, reference the svg files in the rst files,
and keep the .diag files in the doc/source/images/ directory as backup.
Closes-Bug: #2026345
Change-Id: I54cea22e963441b729d4201ad9f8a055a65b54f8