Stop exporting HOSTNAME from scripts

Today we explicitly export the HOSTNAME variable from scripts. This is
probably a relic from the days when the scripts were not run on bash but
instead assume a POSIX-compliant shell only where HOSTNAME is not
guaranteed to exist. Yet, bash guarantees that HOSTNAME is set so we do
not need to set it in scripts. This commit removes this legacy.

Relates elastic/x-pack-elasticsearch#2047

Original commit: elastic/x-pack-elasticsearch@7b833e061c

plugin/bin/x-pack/certgen

@@ -55,8 +55,6 @@ if [ -z "$CONF_DIR" ]; then
     fi
 fi
 
-export HOSTNAME=`hostname -s`
-
 # include x-pack jars in classpath
 ES_CLASSPATH="$ES_CLASSPATH:$ES_HOME/plugins/x-pack/*"
 

plugin/bin/x-pack/croneval

@@ -55,8 +55,6 @@ if [ -z "$CONF_DIR" ]; then
     fi
 fi
 
-export HOSTNAME=`hostname -s`
-
 # include watcher jars in classpath
 ES_CLASSPATH="$ES_CLASSPATH:$ES_HOME/plugins/x-pack/*"
 

plugin/bin/x-pack/extension

@@ -53,8 +53,6 @@ elif [ -f "/etc/default/elasticsearch" ]; then
    . "/etc/default/elasticsearch"
 fi
 
-export HOSTNAME=`hostname -s`
-
 # include x-pack jars in classpath
 ES_CLASSPATH="$ES_CLASSPATH:$ES_HOME/plugins/x-pack/*"
 

plugin/bin/x-pack/migrate

@@ -55,8 +55,6 @@ if [ -z "$CONF_DIR" ]; then
     fi
 fi
 
-export HOSTNAME=`hostname -s`
-
 # include x-pack jars in classpath
 ES_CLASSPATH="$ES_CLASSPATH:$ES_HOME/plugins/x-pack/*"
 

plugin/bin/x-pack/setup-passwords

@@ -55,8 +55,6 @@ if [ -z "$CONF_DIR" ]; then
     fi
 fi
 
-export HOSTNAME=`hostname -s`
-
 # include x-pack jars in classpath
 ES_CLASSPATH="$ES_CLASSPATH:$ES_HOME/plugins/x-pack/*"
 

plugin/bin/x-pack/syskeygen

@@ -55,8 +55,6 @@ if [ -z "$CONF_DIR" ]; then
     fi
 fi
 
-export HOSTNAME=`hostname -s`
-
 # include x-pack jars in classpath
 ES_CLASSPATH="$ES_CLASSPATH:$ES_HOME/plugins/x-pack/*"
 

plugin/bin/x-pack/users

@@ -55,8 +55,6 @@ if [ -z "$CONF_DIR" ]; then
     fi
 fi
 
-export HOSTNAME=`hostname -s`
-
 # include x-pack jars in classpath
 ES_CLASSPATH="$ES_CLASSPATH:$ES_HOME/plugins/x-pack/*"