Remove hardcoded Windows env var for JAVA_HOME

This fixes Azure#421

Author: brunoborges

worker.config.json

@@ -2,8 +2,8 @@
     "description": {
         "language": "java",
         "extensions": [".jar"],
-        "defaultExecutablePath": "%JAVA_HOME%/bin/java",
+        "defaultExecutablePath": "java",
         "defaultWorkerPath": "azure-functions-java-worker.jar",
         "arguments": ["-XX:+TieredCompilation -XX:TieredStopAtLevel=1 -noverify -Djava.net.preferIPv4Stack=true -jar", "%JAVA_OPTS%", "%AZURE_FUNCTIONS_MESH_JAVA_OPTS%"]
     }
-}
+}