Poetry of Programming

The ’svn merge’ command compares two trees, generates a patch, then
applies that patch to a working copy. Yes, you have complete freedom
to compare any two trees, and thereby generate any patch you want. But
that does *not* mean that ’svn merge’ always will do what you want.
It’s *your* responsbility […]

Make sure you are in the root of your app and do:
svn propedit svn:externals vendor/plugins
Just delete the line(s) of the plugins you no longer want.
If you get the following error then you need to set your environment variable.
svn: None of the environment variables SVN_EDITOR, VISUAL or EDITOR is set, and no ‘editor-cmd’ run-time configuration option […]