self: Remove stale comment about "mmap(PROT_NONE) failed" crash.

This crash was fixed by 0aef94e7bc, itself
a followup to 47d48f0c43.

* guix/self.scm (translate-texi-manuals)[build]: Remove reference to the
PROT_NONE bug.

guix/self.scm

@@ -407,9 +407,8 @@ (define (available-translations directory domain)
                                     "\\.[a-z]{2}(_[A-Z]{2})?\\.po$")))
 
           (define parallel-jobs
-            ;; Limit thread creation by 'n-par-for-each'.  Going beyond can
-            ;; lead libgc 8.0.4 to abort with:
-            ;; mmap(PROT_NONE) failed
+            ;; Limit thread creation by 'n-par-for-each', mostly to put an
+            ;; upper bound on memory usage.
             (min (parallel-job-count) 4))
 
           (mkdir #$output)