Unbinding Alt-1 Does Not Work

[Richard Kimberly Heck]

A user reported that many of his custom shortcuts were not working. 
These used Alt-1, etc, as a prefix (e.g., Alt-1 M). Because 2.4 now 
binds Alt-NUM to "Save bookmark NUM", that is not a surprise. What is a 
surprise is that it does not seem possible to unbind this. Even if I 
unbind Alt-9, say, it still saves bookmark 9.

Using -dbg kbmap does not give any useful information. Unbinding does 
not seem to generate any messages.

Riki