chore(scripts): remove unused VERBOSE declarations

Remove VERBOSE="${VERBOSE:-0}" from scripts that never reference
$VERBOSE after setting it. The variable is already set in
scripts/shared.sh line 5.

scripts/install-gh-extensions.sh

@@ -5,9 +5,6 @@ set -euo pipefail
 # shellcheck source="shared.sh"
 source "${DOTFILES}/config/shared.sh"
 
-# Enable verbosity with VERBOSE=1
-VERBOSE="${VERBOSE:-0}"
-
 msgr run "Installing gh (GitHub Client) extensions"
 
 if ! command -v gh &> /dev/null; then