Revert "Making software km implementation both backup and default"

This reverts commit cfc8b73b67b365c3c02b0d8a4ed1c0e7d031d4a9.

Reason for revert: Causing pre-submit failure with window manager
Bug: 148773266
Bug: 149892576

Change-Id: I1acd288aa1a1cb004d1118b7db775511a2cda344
diff --git a/keystore/keystore_main.cpp b/keystore/keystore_main.cpp
index 8e9c699..91ebd12 100644
--- a/keystore/keystore_main.cpp
+++ b/keystore/keystore_main.cpp
@@ -114,6 +114,7 @@
         LOG(WARNING) << "No secure Keymaster implementation found, but device offers insecure"
                         " Keymaster HAL. Using as default.";
         result[SecurityLevel::TRUSTED_ENVIRONMENT] = result[SecurityLevel::SOFTWARE];
+        result[SecurityLevel::SOFTWARE] = nullptr;
     }
     if (!result[SecurityLevel::SOFTWARE]) {
         auto fbdev = android::keystore::makeSoftwareKeymasterDevice();