frr.service: remove explicit dependency on socket-based syslog.target

frr.service: remove explicit dependency on socket-based syslog.target

Testing done: built and installed for all ubuntu targets

Socket-based services such as syslog need not be specified as
dependencies in service files, and doing so may slow down boot by
reducing parallelism.  All known supported systemd-based platforms
have syslog as a socket-based service.

Clears the following lintian warnings:
W: frr: systemd-service-file-refers-to-obsolete-target lib/systemd/system/frr.service syslog.target

Signed-off-by: Silas McCroskey <smccroskey@cumulusnetworks.com>