LogProtoFileWriter: remove explicit lseek() call

syslog-ng 3.4 reintroduced using O_APPEND when opening a destination
file so the explicit lseek() is not needed. This should be a performance
boost and also solves an interoperability problem with Linux /proc files
which do not support seeking.

Reported-By: Valentijn Sessink <valentijn@sessink.nl>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>