OEMCrypto_DeleteUsageTable now returns OEMCrypto_SUCCESS

Copy of widevine change:
https://widevine-internal-review.googlesource.com/#/c/10911/

OEMCrypto_DeleteUsageTable used to return an error on every call
because UsageTable::Clear always returned false.  Since there is no
error checking that Clear can do, its return type has been changed to
void, and DeleteUsageTable now returns OEMCrypto_SUCCESS for all
calls.

bug: 16799906
Change-Id: Iaa2f572e4b0feb554877579596a7f43a64d20954

libwvdrmengine/oemcrypto/mock/src/oemcrypto_usage_table_mock.cpp

@@ -406,13 +406,12 @@ bool UsageTable::DeleteEntry(const std::vector<uint8_t> &pst) {
   return SaveToFile();
 }
 
-bool UsageTable::Clear() {
+void UsageTable::Clear() {
   wvcdm::AutoLock lock(lock_);
   for (EntryMap::iterator i = table_.begin(); i != table_.end(); ++i) {
     if (i->second) delete i->second;
   }
   table_.clear();
-  return false;
 }
 
 bool UsageTable::ComputeHash(const std::vector<uint8_t> &pst,