psppire-var-sheet: Remove unneeded and useless check of unsigned for >= 0.
[pspp] / src /
drwxr-xr-x   ..
-rw-r--r-- 21 .gitignore
-rw-r--r-- 921 automake.mk
drwxr-xr-x - data
drwxr-xr-x - language
drwxr-xr-x - libpspp
drwxr-xr-x - math
drwxr-xr-x - output
drwxr-xr-x - ui