Remove the default value for environment input

We don't want to fill the default value with value that is supported, to
prevent from accidental dispatches of the workflow by people who don't
fully understand how the inputs should be configured. Previously we used
explicitely incorrect default value, but that may be too much and may be
a bit confusing. Let's leave the input without default.

Author: michalinacienciala

.github/workflows/contracts.yaml

@@ -26,7 +26,6 @@ on:
       environment:
         description: "Environment (network) for workflow execution, e.g. `goerli`"
         required: false
-        default: "dev"
       upstream_builds:
         description: "Upstream builds"
         required: false