write_graphite: remove linking against libyajl

[mfournier]

write_graphite doesn't have anything to do with json. This seems to have
been accidentally added in 30c1111.

Fixes https://bugs.debian.org/839771

[mfournier]

FWIW, with this patch we get:

$ /opt/collectd/sbin/collectd  -T
option = ReadThreads; value = -1;
Created new plugin context.
plugin_load: plugin "cpu" successfully loaded.
plugin_load: plugin "write_graphite" successfully loaded.
Done parsing `/opt/collectd/share/collectd/types.db'
interval_g = 10.000;
timeout_g = 2;
hostname_g = lonquimay.wrk.lsn.camptocamp.com;
Exiting normally.
collectd: Stopping 5 write threads.
plugin: stop_write_threads: Signalling `write_cond'
write_graphite plugin: Connecting to localhost:2003 via tcp failed. The last error was: failed to connect to remote host: Connection refused
write_graphite plugin: wg_flush_nolock: timeout = 0.000; send_buf_fill = 0;
$ ldd /opt/collectd/lib/collectd/write_graphite.so 
        linux-vdso.so.1 (0x00007ffd3252e000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f216c469000)
        /lib64/ld-linux-x86-64.so.2 (0x00005572c0416000)

[octo]

LGTM

The write_graphite plugin was building with utils_format_json.c for a long time, though. It appears that was added in 2f6cab9 back in 2012 …