CDM core: Removed support for v15 provisioning responses.

[ Merge of http://go/wvgerrit/160277 ]

OEMCrypto v15 did not require core messages during DRM certificate
provisioning.  The CDM's certificate provisioning flow was allowing
for either case (with or without core messages) when provisioning.

Now, devices can safely assume that all provsisioning responses will
be v16 or newer; all requests/response must contain a core message.

Bug: 252670759
Test: run_x86_64_tests and request_license_test
Change-Id: I9f51e07caf642eaf646ef40bdd640b3ccfe2533c

libwvdrmengine/cdm/core/include/certificate_provisioning.h

@@ -50,8 +50,6 @@ class CertificateProvisioning {
       wvutil::FileSystem* file_system, const CdmProvisioningResponse& response,
       std::string* cert, std::string* wrapped_key);
 
-  bool supports_core_messages() const { return supports_core_messages_; }
-
   // Helper methods
 
   // Extract serial number and system ID from a DRM Device certificate.
@@ -123,13 +121,6 @@ class CertificateProvisioning {
   // Key type of the generated key pair in provisioning 4.
   CryptoWrappedKey::Type provisioning_40_key_type_;
 
-  // Indicates whether OEMCrypto supports core messages, and whether the
-  // CDM should expect a core message in the response.  This is primarily
-  // used to distinguish between v16+ OEMCrypto or an earlier version.
-  // Assume core messages are supported, and check if OEMCrypto populates
-  // the core message field when calling PrepAndSignProvisioningRequest().
-  bool supports_core_messages_ = true;
-
   CORE_DISALLOW_COPY_AND_ASSIGN(CertificateProvisioning);
 };