tox: Remove bst-plugins-experimental dep

tox.ini

diff --git a/tox.ini b/tox.ini
index c6bcd78..d29d429 100644
--- a/tox.ini
+++ b/tox.ini
@@ -30,7 +30,6 @@
     py{35,36,37}: -rrequirements/requirements.txt
     py{35,36,37}: -rrequirements/dev-requirements.txt
     py{35,36,37}: -rrequirements/plugin-requirements.txt
-    git+https://gitlab.com/BuildStream/bst-plugins-experimental.git@effd6be61b6ed5983694da7496adf872d81eddcc
 
     # Only require coverage and pytest-cov when using it
     !nocover: -rrequirements/cov-requirements.txt