Stop installing PHPCS globally in the Travis build job

PHPCS is already installed as a project dependency (we run `composer
install` for each build job) so there is no need to install it as a
global package in `tests/bin/phpcs.sh`.

tests/bin/phpcs.sh

@@ -5,11 +5,6 @@ if [[ ${RUN_PHPCS} == 1 ]]; then
 	IGNORE="tests/cli/,includes/libraries/,includes/api/legacy/"
 
 	if [ "$CHANGED_FILES" != "" ]; then
-		if [ ! -f "./vendor/bin/phpcs" ]; then
-			# Install wpcs globally
-			composer global require woocommerce/woocommerce-sniffs --update-with-all-dependencies
-		fi
-
 		echo "Running Code Sniffer."
 		vendor/bin/phpcs --ignore=$IGNORE --encoding=utf-8 -s -n -p $CHANGED_FILES
 	fi