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>