Remove unlabelled stack.yaml and ghc-default job

This stack descriptor was a duplicate of the 8.6.5 descriptor.

No reason to have 2 copies and two CI jobs of the same configuration

Author: pepeiborra

.circleci/config.yml

@@ -125,11 +125,6 @@ jobs:
       - STACK_FILE: "stack-8.10.3.yaml"
     <<: *defaults
 
-  ghc-default:
-    environment:
-      - STACK_FILE: "stack.yaml"
-    <<: *defaults
-
 
 workflows:
   version: 2
@@ -143,4 +138,3 @@ workflows:
       - ghc-8.10.1
       - ghc-8.10.2
       - ghc-8.10.3
-      - ghc-default