Remove explicit set of WorkingDirectory

Remove explicit set of WorkingDirectory

In the systemd unit for FS/debian, if the WorkingDirectory is not set
then it defaults to '/'.  This is fine for FS, and is a common and
expected chdir choice for daemons.

We had previously set this to /run/freeswitch.  Due to Debian having
systemd-coredump(8) disabled, this was causing core files to be
written to /run/freeswitch, which is a bad place for them as it's
mounted on tmpfs.

It's better to leave FS as running on '/'.  This will prevent core
files from being written unless the user adjusts `sysctl
kernel.core_pattern`, which is a reasonable thing to expect if the
user wants these files.  Core files can be huge, and having them go
anywhere unexpectedly can be a problem.

When Debian adds the systemd-coredumps support this will all work
nicely and automatically.

ref: http://www.freedesktop.org/software/systemd/man/coredump.conf.html
ref: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=744964

FS-7909