title: typeorm won't allow dropping index on abstract class
body: <p>I have an abstract class like so,</p>
  <pre><code>@Entity('blob_instance')
  @TableInheritance({ column: { name: 'blobType', type: 'enum' } })
  export abstract class BlobInstance {
    @PrimaryGeneratedColumn('uuid')
    readonly blobInstanceId!: string;
  
    /*
     * TypeORM automatically generates a new enum type in postgres for this column.
     * While TypeORM should allow overriding the type with enumName (https://github.com/typeorm/typeorm/issues/4106),
     * this feature does not currently work with postgres, causing duplicate migrations (https://github.com/typeorm/typeorm/issues/6471)
     */
    @Column({
      type: 'enum',
      enum: BlobType,
    })
    abstract blobType: BlobType;
  </code></pre>
  <p>Typeorm automatically generates an index on <code>blobType</code>. I have tried manually dropping it. But every time i try to create a new migration it comes up. Is there a way to signal that I don't want this index on <code>blobType</code>?</p>