Remove suggestions of using commit number

Using a commit number is trickier than it sounds. It would need to be
padded to sort correctly, for one thing. It is better to leave it out
than to give an incomplete account.

Signed-off-by: Michael Bridgen <michael@weave.works>

docs/guides/sortable-image-tags.md

@@ -30,17 +30,6 @@ Alternatively, you can use a stable serial number as part of the tag.  Some CI p
 provide a build number in an environment variable, but that may not be reliable to use as a serial
 number -- check the platform documentation.
 
-A commit count can be a reasonable stand-in for a serial number, if you build an image per commit,
-and you don't rewrite the branch in question.
-
-```bash
-$ # commits in branch
-$ git --rev-list --count HEAD
-1504
-```
-
-Beware: this will not give a useful number if you have a shallow clone.
-
 ### Other things to include in the image tag
 
 It is also handy to quickly trace an image to the branch and commit of its source code. Including