lvm: remove duplicate call to lvm_submit()

This got added by accident when solving the merge conflict in 103f05e0.

It led to the plugin triggering the classical "uc_update: Value too old"
error message.
Merge pull request #791 from mfournier/fix-lvm-merge-conflict-mistake

lvm: remove duplicate call to lvm_submit()
kafka: fix compilation for older versions of librdkafka

Since commit f505691270f2317291c372fd5f004a4ffbce9f9a, kafka module was
broken. Enable definition of `kafka_log()` when using kafka logger
callback as well.
Merge pull request #794 from vincentbernat/fix/kafka-warning

kafka: fix compilation for older versions of librdkafka