Use isJust instead of reimplementing it

src/ShellCheck/Checks/ShellSupport.hs

@@ -456,11 +456,10 @@ checkEchoSed = ForShell [Bash, Ksh] f
 
     -- This should have used backreferences, but TDFA doesn't support them
     sedRe = mkRegex "^s(.)([^\n]*)g?$"
-    isSimpleSed s = fromMaybe False $ do
+    isSimpleSed s = isJust $ do
         [h:_,rest] <- matchRegex sedRe s
         let delimiters = filter (== h) rest
         guard $ length delimiters == 2
-        return True
     checkIn id s =
         when (isSimpleSed s) $
             style id 2001 "See if you can use ${variable//search/replace} instead."