llava : add --skip-unknown to 1.6 convert.py (#5632)

This commit adds the `--skip-unknown` option to the convert.py script
and removes the saving of the updated checkpoints to avoid updating
possibly checked out files.

The motivation for this change is that this was done for 1.5
in Commit fc0c8d286a533363a9a663510b62af85ffad58b3 ("llava :
update surgery script to not remove tensors") and makes the examples
more consistent.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>