Merge pull request #4795 from WoltLab/mb-strpos

Remove unneeded usage of `mb_strpos`

diff --cc wcfsetup/install/files/lib/page/AbstractPage.class.php
Simple merge

diff --cc wcfsetup/install/files/lib/system/WCFSetup.class.php
Simple merge

diff --cc wcfsetup/install/files/lib/system/template/TemplateScriptingCompiler.class.php
Simple merge

diff --cc wcfsetup/install/files/lib/util/HeaderUtil.class.php
Simple merge