Mute XCache error when trying to clear the opcode cache

From https://github.com/owncloud/core/issues/16287:

> This is caused by XCache at https://github.com/owncloud/core/blob/8e59d4c64b48ed7daeebb714e11213f7a22b5740/lib/private/util.php#L1276 where we are trying to reset the opcode cache with `XC_TYPE_PHP`.
> I suspect that while XCache is installed its opcode component is not used. Unfortunately, the XCache API is not really properly documented and thus I don't know what API we would have to call to check whether the `XC_TYPE_PHP` cache is populated. In fact, there is an [open XCache bug](http://xcache.lighttpd.net/ticket/176) since 7 years that discusses this problem and is likely to never get fixed since XCache is abandonware.

Fixes https://github.com/owncloud/core/issues/16287