Remove syntaxCheck setting - suppress warning

According to Sebastian Bergmann: "That configuration setting has not had an effect in years. It was removed long ago".

phpunit.xml

@@ -7,7 +7,6 @@
          convertWarningsToExceptions="true"
          processIsolation="false"
          stopOnFailure="false"
-         syntaxCheck="false"
          bootstrap="./tests/bootstrap.php">
     <testsuites>
         <testsuite name="Unit tests">