sysctl: Enable tcp timestamps for improved tcp performance

tcp timestamps can improve tcp performance a lot, especially
with unstable networks like wifi.

Linux default behaviour is enabled so the tcp timestamps setting
is removed from sysctl configuration.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Christian Dresel <fff@chrisi01.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>