Remove plonematch theme from bioformats repository

Sphinx theme for the HTML documentation has been moved to a standalone
repository https://github.com/openmicroscopy/sphinx_theme and is used to
generate documentation in the CI builds.

Only impact of this commit on local builds is that "-D html_theme=plonematch"
can not be used in the SPHINXOPTS envvar anymore.
Remove deprecated plone target in Sphinx Makefile
Merge pull request #593 from sbesson/theme_cleanup

Remove plonematch theme from Bio-Formats documentation