Remove dead code from "git am"

Ever since the initial implementation, "git am" had kept a dead code that
never triggered due to a typo in the variable name.  Worse yet, the code,
if it weren't for the typo, would have attempted to add "[PATCH] " at the
beginning of the Subject: header when "git am" is run with its "-k"
option.  However, because "git am -k" tells mailinfo to keep such prefix
when parsing the input, the "[PATCH] " added by this dead code would have
really been unnecessary duplicate.

Embarrassing is that we kept _maintaining_ the codepath without anybody
noticing for four years.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -350,11 +350,12 @@ do
 			git cat-file commit "$commit" |
 			sed -e '1,/^$/d' >"$dotest/msg-clean"
 		else
-			SUBJECT="$(sed -n '/^Subject/ s/Subject: //p' "$dotest/info")"
-			case "$keep_subject" in -k)  SUBJECT="[PATCH] $SUBJECT" ;; esac
-
-			(printf '%s\n\n' "$SUBJECT"; cat "$dotest/msg") |
-				git stripspace > "$dotest/msg-clean"
+			{
+				sed -n '/^Subject/ s/Subject: //p' "$dotest/info"
+				echo
+				cat "$dotest/msg"
+			} |
+			git stripspace > "$dotest/msg-clean"
 		fi
 		;;
 	esac