[ci] Remove mustache_template from allowed external deps (#10427)

mustache_template was moved into this repository, so is no longer an external dependency that requires an exception.

Author: stuartmorgan-g

script/configs/allowed_pinned_deps.yaml

@@ -19,7 +19,3 @@
 # audited before passing them on to clients as transitive updates, to mitigated
 # the risk of the package being compromised.
 - xml
-
-# Already used by flutter_tools, so doesn't add any additional team-level
-# maintenance risk. Could require forking in the future.
-- mustache_template