Revert "Connect plugins on start. Closes #2411"

__tests__/src/extend/pluginPreprocessing.test.js

__tests__/src/extend/pluginStore.test.js

__tests__/src/extend/withPlugins.test.js

__tests__/src/lib/MiradorViewer.test.js

src/extend/index.js

src/extend/pluginContext.js

src/extend/pluginProvider.js

src/extend/pluginStore.js

src/extend/withPlugins.js

src/lib/MiradorViewer.js