mhbuild: Removed -contentid switch, which was an Outlook 2002 workaround.

mhbuild: Removed -contentid switch, which was an Outlook 2002 workaround.
Explanation:
Outlook 2002 won't display attachments that have a Content-ID header.
This is a bug in Outlook 2002, not in mhbuild. There used to be this
-nocontentid switch to prevent Content-ID headers to be inserted. As I
consider it wrong to complicate all other MUAs instead of forcing the
developers and users of broken MUAs to fix or change their software,
I removed it.
You can still workaround the bug in the following way: invoke
`e mhbuild' manually at the Whatnow prompt and edit the draft again
thereafter, removing the Content-ID headers. Then send it.