I found out the only way to "remove" the text is to replace it. Spaces don't count, so if I don't want anything in the field, just a . or - would work. I tested it in a new, blank FM file, so it seems to be applicable to all FM files. Not sure if this is by design, or a bug. 
This is a bug.
The information is stored in two locations in the MIF:
- in the <PDFDocInfo ... > # end of PDFDocInfo
- in the <DocFileInfo ... > # end of DocFileInfo.
To delete a value, you must remove it from both locations in the MIF. I'd suggest that you delete the entire DocFileInfo. Then edit the PDFDocInfo so that it contains only those values that you want to keep.
Please also report this as a bug at https://bugbase.adobe.com/index.cfm .
FYI, the DocFileInfo is hexadecimal-encoded XML. After decoding, it looks like this:
<?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?>
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 5.4-c005 78.150055, 2012/11/19-18:45:32 ">
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf:Description rdf:about=""
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:xmp="http://ns.adobe.com/xap/1.0/"
xmlns:pdf="http://ns.adobe.com/pdf/1.3/">
<dc:format>application/framemaker</dc:format>
<dc:description>
<rdf:Alt>
<rdf:li xml:lang="x-default">DELETE THIS</rdf:li>
</rdf:Alt>
</dc:description>
<xmp:CreatorTool>FrameMaker 10.0</xmp:CreatorTool>
<xmp:CreateDate>0116-03-12T10:24:38+01:00</xmp:CreateDate>
<xmp:MetadataDate>0116-03-12T10:26:01+01:00</xmp:MetadataDate>
<xmp:ModifyDate>0116-03-12T10:26:01+01:00</xmp:ModifyDate>
<pdf:CreatorTool>FrameMaker 10.0</pdf:CreatorTool>
<pdf:Keywords>DELETE THIS</pdf:Keywords>
</rdf:Description>
</rdf:RDF>
</x:xmpmeta>
<?xpacket end="w"?>
I suspect that while you edit the document, FrameMaker is using only one of these structures (PDFDocInfo/DocFileInfo). But when you save the document, it is then merging them, copying data from the unused structure if the other structure is empty.