CLEANUP: log: removing "log-balance" references

CLEANUP: log: removing "log-balance" references

This is a complementary patch to 0e1f389fe9 ("DOC: config: removing
"log-balance" references"): we properly removed all log-balance
references in the doc but there remained some in the code, let's fix
that.

It could be backported in 2.9 with 0e1f389fe9

(cherry picked from commit ddf66132f4d77e00c52c977d8a9e5c829965e7c7)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 24160712d38f54fe4114379ecbe034bcb25ee976)
[ad: ctx adjustment]
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>