core: Don't account RAM metadata to RAM quota

The meta data for allocations should be accounted to the session quota
instead of the RAM accont managed by the session.

base/src/core/include/ram_session_component.h

@@ -73,8 +73,7 @@ namespace Genode {
 			/**
 			 * Return portion of RAM quota that is currently in use
 			 */
-			size_t used_quota() {
-				return _ds_slab.consumed() + _payload + sizeof(*this); }
+			size_t used_quota() { return _payload; }
 
 			/**
 			 * Free dataspace