minor readability change

As suggested by @sogaiu

@zevv forget to push this change in a recent PR (janet-lang#1175 (comment)).

Incidentally, the affected lines were already reformatted in the current PR, via fmt/format-file.

Author: primo-ppcg

src/boot/boot.janet

@@ -4189,12 +4189,11 @@
 
   (defn do-one-file
     [fname]
-    (if-not (has-value? boot/args "image-only")
-      (do
-        (print "\n/* " fname " */")
-        (print "#line 0 \"" fname "\"\n")
-        (def source (slurp fname))
-        (print (string/replace-all "\r" "" source)))))
+    (unless (has-value? boot/args "image-only")
+      (print "\n/* " fname " */")
+      (print "#line 0 \"" fname "\"\n")
+      (def source (slurp fname))
+      (print (string/replace-all "\r" "" source))))
 
   (do-one-file feature-header)