Revert external ID cache

It make more sense to include the commit SHA-1 of the notes branch from
which the external IDs have been read into the cache key. This makes it
easier to implement this cache within a multimaster setup.
However, obviously, we can include the commit SHA-1 of the notes branch
into the cache key only after the external IDs have been migrated to
NoteDb (because before this the notes branch does not exist yet).
Revert the cache for now and re-add an improved version of the cache
later with the change that implements the migration of the external IDs
to NoteDb.

This reverts the following commits:
- 2869caaf70:
  Add cache for external ids
- 48d5c9b9aa:
  Make ExternalIdCacheImpl.AllKey public
- 20e5507d46:
  ExternalIdCache: Add method to get external IDs by account ID + scheme

Change-Id: I589242aad32a9c70718542ba950c0a351c594e54
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/AccountExternalId.java

@@ -18,8 +18,6 @@ import com.google.gerrit.extensions.client.AuthType;
 import com.google.gwtorm.client.Column;
 import com.google.gwtorm.client.StringKey;
 
-import java.util.Objects;
-
 /** Association of an external account identifier to a local {@link Account}. */
 public final class AccountExternalId {
   /**
@@ -169,21 +167,4 @@ public final class AccountExternalId {
   public void setCanDelete(final boolean t) {
     canDelete = t;
   }
-
-  @Override
-  public boolean equals(Object o) {
-    if (o instanceof AccountExternalId) {
-      AccountExternalId extId = (AccountExternalId) o;
-      return Objects.equals(key, extId.key)
-          && Objects.equals(accountId, extId.accountId)
-          && Objects.equals(emailAddress, extId.emailAddress)
-          && Objects.equals(password, extId.password);
-    }
-    return false;
-  }
-
-  @Override
-  public int hashCode() {
-    return Objects.hash(key, accountId, emailAddress, password);
-  }
 }