src/net/proxy: remove unused variable
src/net/proxy: remove unused variable

clang 15 emits unused-but-set-variable warning for variable i in loop (i doesn't occur in the body):

for (size_t i = 0; !p.empty() && _send_depth < _send_queue_length; i++, _send_depth++) {

Closes #1075

scylladb/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar