Remove testing of molecule-digitalocean

While trying to fix some conflicts caused by this driver, I discovered
that it did not receive any active maintenance in almost two years.

Related: ansible-community/molecule-digitalocean#3 (comment)

tox.ini

@@ -187,7 +187,7 @@ deps =
     ansible-core >= 2.11
     molecule-azure >= 0.5.0
     molecule-containers >= 1.0.0
-    molecule-digitalocean >= 0.1
+    # molecule-digitalocean >= 0.1  # removed on 2021-11-26 (unmaintained)
     molecule-docker >= 1.0.2
     molecule-ec2 >= 0.3
     molecule-gce >= 0.2