remove never used phpunit configuration (#192)

regarding this answer from phpunit's maintainer, the configuration value has never been used:
https://stackoverflow.com/a/44331140

Author: maglnet

phpunit.xml.dist

@@ -7,8 +7,7 @@
          convertNoticesToExceptions="true"
          convertWarningsToExceptions="true"
          processIsolation="false"
-         stopOnFailure="false"
-         syntaxCheck="false">
+         stopOnFailure="false">
     <testsuites>
         <testsuite name="WebPush Test Suite">
             <directory suffix=".php">./tests/</directory>