Do not close CDM session on removeKeys

A change was introduced between jb-mr2 and klp-dev that closed
the CDM session when the removeKeys mediaDrm APIs was called.
This was introduced because there is no way of unloading keys from
an OEMCrypto session.

This caused problems for Netflix, as an exception occurred when they
tried to close a session after calling removeKeys.

Reverting to jb-mr2 behaviour for now. b/11188818 has been opened to
track a longer term resolution.

b/11185042

Change-Id: I03bc736d2bc5bdabc86cfb7b75ae8bcbc03ffc7c

libwvdrmengine/cdm/core/src/cdm_engine.cpp

@@ -295,8 +295,6 @@ CdmResponseType CdmEngine::CancelKeyRequest(const CdmSessionId& session_id) {
   }
 
   // TODO(edwinwong, rfrias): unload keys here
-  delete iter->second;
-  sessions_.erase(session_id);
   DisablePolicyTimer();
   return NO_ERROR;
 }