When I define an Inheritance (Single Table) on an entity, then remove it as well as all the child entities and the type column, there is still the definition left over and exported. I cannot delete it in ORM Designer, I have to manually edit the ORMD XML in a text editor.
See below for ORMD model before and after deletion as well as a screenshot:

Model Version 1:
<?xml version="1.0"?>
<orm-designer version="2.2.4.749" name="Ormd2" mvc="Symfony2" orm="Doctrine2" uuid="d76ad543-e605-48a5-95bb-de75d6de896d">
<module name="MainModule" uuid="fde41f8b-f883-40d4-9cda-6adfae7389fd">
<entity name="OtherEntity" uuid="a86a712c-6d35-4586-a880-324b9a66098a">
<inheritance-child derived-from="SampleEntity" discriminator-value="OtherEntity" caption="New inheritance" uuid="aaccb6af-9180-432b-8ebc-34506875c9a8"/>
</entity>
<entity name="SampleEntity" uuid="1db8bf02-4bcd-4232-8305-17af4c80bd74">
<field name="id" type="integer" required="true" unique="true" primary="true" auto-increment="true" uuid="79f7f8d4-c26e-492f-9281-f2d0f531c6d5"/>
<field name="name" type="string" size="255" uuid="e3590f9c-f052-47fe-ad20-8fb009646e49"/>
<field name="type" type="string" required="true" uuid="6e8f4578-c03a-4f28-b112-397857f301c6"/>
<inheritance-parent discriminator-field="type" type="SINGLE_TABLE" uuid="4f338887-215f-4357-9b49-e1b10a3c9b7e"/>
</entity>
</module>
<visual-data>
<entity uuid="1db8bf02-4bcd-4232-8305-17af4c80bd74" position-x="10" position-y="20"/>
<entity uuid="a86a712c-6d35-4586-a880-324b9a66098a" position-x="121" position-y="93" size-x="0" size-x2="20" size-y="0" size-y2="20"/>
<inheritance-child uuid="aaccb6af-9180-432b-8ebc-34506875c9a8" caption1-position-x="0" caption1-position-y="0" center-position-x="0" center-position-y="0"/>
<project uuid="d76ad543-e605-48a5-95bb-de75d6de896d" size-x="0" size-x2="331" size-y="0" size-y2="285"/>
<module uuid="fde41f8b-f883-40d4-9cda-6adfae7389fd" bk-color="4294375930" position-x="61" position-y="72" size-x="11" size-x2="220" size-y="22" size-y2="163"/>
</visual-data>
</orm-designer>
Model Version 2:
<?xml version="1.0"?>
<orm-designer version="2.2.4.749" name="Ormd2" mvc="Symfony2" orm="Doctrine2" uuid="d76ad543-e605-48a5-95bb-de75d6de896d">
<module name="MainModule" uuid="fde41f8b-f883-40d4-9cda-6adfae7389fd">
<entity name="SampleEntity" uuid="1db8bf02-4bcd-4232-8305-17af4c80bd74">
<field name="id" type="integer" required="true" unique="true" primary="true" auto-increment="true" uuid="79f7f8d4-c26e-492f-9281-f2d0f531c6d5"/>
<field name="name" type="string" size="255" uuid="e3590f9c-f052-47fe-ad20-8fb009646e49"/>
<inheritance-parent type="SINGLE_TABLE" uuid="4f338887-215f-4357-9b49-e1b10a3c9b7e"/>
</entity>
</module>
<visual-data>
<entity uuid="1db8bf02-4bcd-4232-8305-17af4c80bd74" position-x="10" position-y="20"/>
<project uuid="d76ad543-e605-48a5-95bb-de75d6de896d" size-x="0" size-x2="331" size-y="0" size-y2="285"/>
<module uuid="fde41f8b-f883-40d4-9cda-6adfae7389fd" bk-color="4294375930" position-x="61" position-y="72" size-x="11" size-x2="220" size-y="22" size-y2="163"/>
</visual-data>
</orm-designer>