Removed now useless BACKEND_URL setting

.env.dev

-BACKEND_URL=http://localhost:6001
 API_AUTHENTICATION_REQUIRED=True
 CACHALOT_ENABLED=False
 RAVEN_ENABLED=false

front/config/prod.env.js

 module.exports = {
   NODE_ENV: '"production"',
-  BACKEND_URL: '"' + (process.env.BACKEND_URL  || '/') + '"'
+  BACKEND_URL: '"/"'
 }

front/src/config.js

 class Config {
   constructor () {
     this.BACKEND_URL = process.env.BACKEND_URL
-    if (this.BACKEND_URL === '/') {
-      this.BACKEND_URL = window.location.protocol + '//' + window.location.hostname + ':' + window.location.port
-    }
-    if (this.BACKEND_URL.slice(-1) !== '/') {
-      this.BACKEND_URL += '/'
-    }
     this.API_URL = this.BACKEND_URL + 'api/v1/'
   }
 }