CI: disable codecov project status.

Every PR should be covered by tests, and codecov patch statuses are
extremely useful. However, codecov project statuses mostly create
noise because project-wide coverage in nMigen is currently primarily
informational.

Author: whitequark

.codecov.yml

@@ -0,0 +1,4 @@
+coverage:
+  status:
+    project:
+      default: false