Removed unused leash help file

The file 'checkbox blank.png' is unneeded and contains a space in the
name, unnecessarily interfering with running find | xargs over the
source tree.  Remove it.

(cherry picked from commit 757860b48bcef42a9a5fddba984607edeb6e205c)

https://github.com/krb5/krb5/commit/d597675e5817beef1d37e0054ebd3b786c33c9b0
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: d597675e5817beef1d37e0054ebd3b786c33c9b0
Branch: krb5-1.10
 .../leash/htmlhelp/Images/checkbox blank.png       |  Bin 1326 -> 0 bytes
 1 files changed, 0 insertions(+), 0 deletions(-)