Remove test subcommand from CLI web plugin

The OMERO.web test infrastructure has been redesigned and now properly lives
under components/tools/OmeroWeb/. This subcommand is not used anywhere and
certainly confusing for production use.

OME-DCO-1.1-Signed-off-by: Sebastien Besson <seb.besson@gmail.com> (github: sbesson)
Merge pull request #2731 from sbesson/omero_cli_webtest

Remove test subcommand from CLI web plugin