Drop unused gpgsign cspell entry

Orphaned when the tool moved off the command-runner design that used
git -c commit.gpgsign=false; the word is no longer referenced anywhere.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: l0lawrence

cspell.yaml

@@ -98,7 +98,6 @@ words:
   - getpgid
   - ghapp
   - giacamo
-  - gpgsign
   - graalvm
   - Gson
   - Hdvcmxk