Remove bashisms for mx4j tool in cassandra-env.sh

/bin/sh (our shebang in bin/cassandra) does not recognize the
`if` construct which is available in /bin/bash only hence if
MX4J_ADDRESS or MX4J_PORT was not commented out, the script would
fail to execute it.

patch by Stefan Miklosovic; reviewed by Brandon Williams for CASSANDRA-19416
2 files changed