tests: drop plugin deps from plugins_siteless

The plugin sets are found by load_all_plugins() walking the cmk.plugins
namespace, and the suite runs as plain pytest via tests/run_tests.sh, so no
runfiles tree is built and the deps supply nothing. Nothing imports the plugin
modules by name either.

Change-Id: I04842623e58926d66b88fb51b54d9254e11f850e