CI: remove installation of mbstring polyfill in build script, it's required dependency now

dev-tools/build.sh

@@ -4,9 +4,6 @@ set -eu
 # ensure that deps will work on lowest supported PHP version
 composer config platform.php 2> /dev/null || composer config platform.php 7.2.5
 
-# require suggested packages
-composer require --no-update symfony/polyfill-mbstring
-
 # install package deps without dev-deps / remove already installed dev-deps
 # box can ignore dev-deps, but dev-deps, when installed, may lower version of prod-deps
 composer update --no-interaction --no-progress --no-dev --prefer-stable --optimize-autoloader