Context: Remove unused BEM field

Can't find any usage of this field. Is it safe to remove?

cc @veged @indutny

lib/bemhtml/runtime/context.js

@@ -35,8 +35,6 @@ Context.prototype.xmlEscape = utils.xmlEscape;
 Context.prototype.attrEscape = utils.attrEscape;
 Context.prototype.jsAttrEscape = utils.jsAttrEscape;
 
-Context.prototype.BEM = {};
-
 Context.prototype.isFirst = function isFirst() {
   return this.position === 1;
 };