Avoid unnecessarily complex git macro for 'git k'

dotfiles/config/gittum/main.gitconfig

         --pretty=tformat:"%x1b[31m%h%x20%x1b[32m%d%x1b[0m%x20%s%x1b[0m"\
         --simplify-by-decoration
   k = !sh -c \
-      '(which gitk >/dev/null || (echo "gitk not installed" && false)) && gitk $@' --
+      'gitk $@' --
   ka = !sh -c \
-       '(which gitk >/dev/null || (echo "gitk not installed" && false)) && gitk $@ --all' --
+       'gitk $@ --all' --
   rb = rebase
   rbc = rebase --continue
   rbi = rebase --interactive