test: repl-persistent-history is no longer flaky

The race conditions were fixed in
286ef1daca

PR-URL: https://github.com/nodejs/node/pull/3437
Reviewed By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

test/sequential/sequential.status

@@ -5,7 +5,6 @@ prefix sequential
 # sample-test                       : PASS,FLAKY
 
 [true] # This section applies to all platforms
-test-repl-persistent-history                    : PASS,FLAKY
 
 [$system==win32]