Remove unsupported environment variables check

The Elasticsearch startup scripts contain checks for the presence of
support for environment variables that were removed in the 5.x
series. These checks warn the user and fail the script if any of the
unsupported environment variables are present. This was provided as
migration step from 2.x to 5.x so that we were not just silently
ignoring environment variables that were previously set. This commit
removes these checks, as upgrades from 2.x to 6.x are not supported.

Relates #20404

distribution/src/main/resources/bin/elasticsearch

@@ -54,35 +54,6 @@ EOF
     exit 1
 fi
 
-# TODO: remove for Elasticsearch 6.x
-unsupported_environment_variable() {
-    if test -n "$1"; then
-        echo "$2=$1: $3"
-    fi
-}
-
-if test -n "$ES_MIN_MEM" ||
-   test -n "$ES_MAX_MEM" ||
-   test -n "$ES_HEAP_SIZE" ||
-   test -n "$ES_HEAP_NEWSIZE" ||
-   test -n "$ES_DIRECT_SIZE" ||
-   test -n "$ES_USE_IPV4" ||
-   test -n "$ES_GC_OPTS" ||
-   test -n "$ES_GC_LOG_FILE"; then
-    echo "Error: encountered environment variables that are no longer supported"
-    echo "Use jvm.options or ES_JAVA_OPTS to configure the JVM"
-    unsupported_environment_variable "$ES_MIN_MEM" ES_MIN_MEM "set -Xms$ES_MIN_MEM in jvm.options or add \"-Xms$ES_MIN_MEM\" to ES_JAVA_OPTS"
-    unsupported_environment_variable "$ES_MAX_MEM" ES_MAX_MEM "set -Xmx$ES_MAX_MEM in jvm.options or add \"-Xmx$ES_MAX_MEM\" to ES_JAVA_OPTS"
-    unsupported_environment_variable "$ES_HEAP_SIZE" ES_HEAP_SIZE "set -Xms$ES_HEAP_SIZE and -Xmx$ES_HEAP_SIZE in jvm.options or add \"-Xms$ES_HEAP_SIZE -Xmx$ES_HEAP_SIZE\" to ES_JAVA_OPTS"
-    unsupported_environment_variable "$ES_HEAP_NEWSIZE" ES_HEAP_NEWSIZE "set -Xmn$ES_HEAP_NEWSIZE in jvm.options or add \"-Xmn$ES_HEAP_SIZE\" to ES_JAVA_OPTS"
-    unsupported_environment_variable "$ES_DIRECT_SIZE" ES_DIRECT_SIZE "set -XX:MaxDirectMemorySize=$ES_DIRECT_SIZE in jvm.options or add \"-XX:MaxDirectMemorySize=$ES_DIRECT_SIZE\" to ES_JAVA_OPTS"
-    unsupported_environment_variable "$ES_USE_IPV4" ES_USE_IPV4 "set -Djava.net.preferIPv4Stack=true in jvm.options or add \"-Djava.net.preferIPv4Stack=true\" to ES_JAVA_OPTS"
-    unsupported_environment_variable "$ES_GC_OPTS" ES_GC_OPTS "set ${ES_GC_OPTS// / and } in jvm.options or add \"$ES_GC_OPTS\" to ES_JAVA_OPTS"
-    unsupported_environment_variable "$ES_GC_LOG_FILE" ES_GC_LOG_FILE "set -Xloggc:$ES_GC_LOG_FILE in jvm.options or add \"-Xloggc:$ES_GC_LOG_FILE\" to ES_JAVA_OPTS"
-    exit 1
-fi
-# end TODO: remove for Elasticsearch 6.x
-
 parse_jvm_options() {
   if [ -f "$1" ]; then
     echo "$(grep "^-" "$1" | tr '\n' ' ')"