disable verbose logging for xserver, fixes #4333

The current options for the xserver produce a huge amount of log messages. The
server produces around 70-80 messages per minute. The most messages look like
this:

display-manager-start[1846]: GetModeLine - scrn: 0 clock: 75200
display-manager-start[1846]: GetModeLine - hdsp: 1366 hbeg: 1414 hend: 1478 httl: 1582
display-manager-start[1846]: vdsp: 768 vbeg: 772 vend: 779 vttl: 792 flags: 9

Since theses messages aren't very usefull, I propose to remove the `-logverbose`
and `-verbose` options from the xserver arguments.