Deal with shellcheck warnings in 1993/plummer

This was only in try.sh: SLEEP is not needed which is why the error was
not detected. SLEEP is only relevant to the alt code. But the try.alt.sh
was updated to make it shorter for assigning the default:

    [[ -z "$SLEEP" ]] && SLEEP=1000

1000 is the default in the Makefile.

1993/plummer/try.alt.sh

@@ -12,9 +12,7 @@ fi
 
 # let user change the sleep duration if they wish
 #
-if [[ -z "$SLEEP" ]]; then
-    SLEEP=1000
-fi
+[[ -z "$SLEEP" ]] && SLEEP=1000
 
 # let user change the number if they wish to
 #

1993/plummer/try.sh

@@ -26,7 +26,7 @@ else
     ARG="$2"
 fi
 
-make CC="$CC" SLEEP="$SLEEP" all >/dev/null || exit 1
+make CC="$CC" all >/dev/null || exit 1
 
 # clear screen after compilation so that only the entry is shown
 clear