BUG/MINOR: backend: don't use url_param_name as a hint for BE_LB_ALGO_PH

BUG/MINOR: backend: don't use url_param_name as a hint for BE_LB_ALGO_PH

At a few places in the code we used to rely on this variable to guess
what LB algo was in place. This is wrong because if the defaults section
presets "balance url_param foo" and a backend uses "balance roundrobin",
these locations will still see this url_param_name set and consider it.
The harm is limited, as this only causes the beginning of the request
body to be buffered. And in general this is a bad practice which prevents
us from cleaning the lbprm stuff. Let's explicitly check the LB algo
instead.

This may be backported to all currently maintained versions.