This should work:
sed -i .bak 's/[^:]*: *//' file
-i .bak will edit the file in place, and create a backup of the original called
s/pat/replacement/ means substitute
[^:]*: * means match the longest string of non-
: characters, followed by a
: then one or more space. The final result is that it will delete everything up to the first
: and following spaces.
This approach has the advantage of working with
foo: or whatever and will also work if you have multiple
: on the same line. For example, it can deal with this:
data: a line that contains : a colon!