[Chore] Cleanup stack.yaml

1. The project no longer uses `niv`, so the comment about it is
irrelevant. Dependencies are normally updated by the update-daemon
nowadays.
2. cmark-gfm is available in lts-21.13.

stack.yaml

@@ -2,15 +2,12 @@
 #
 # SPDX-License-Identifier: MPL-2.0
 
-# To update hackage and stackage indexes used by CI run:
-# $ niv update hackage.nix; niv update stackage.nix
 resolver: lts-21.13
 
 packages:
 - .
 
 extra-deps:
 - firefly-0.2.1.0@sha256:e9d73486464c3e223ec457e02b30ddd5b550fdbf6292b268c64581e2b07d888b,1519
-- cmark-gfm-0.2.5
 - nyan-interpolation-core-0.9.2
 - nyan-interpolation-0.9.2