Fix Dependabot by dropping supprt for requirements-dev.txt (#461)

Dependabot fails with this:

Update configs must have a unique combination of 'package-ecosystem',
'directory', and 'target-branch'. Ecosystem 'pip' has overlapping
directories.

.github/dependabot.yml

@@ -8,9 +8,3 @@ updates:
       - "/tests"
     schedule:
       interval: "weekly"
-
-  - package-ecosystem: "pip"
-    directory: "/"
-    file: "requirements-dev.txt"
-    schedule:
-      interval: "weekly"