Remove dead code from "git am"

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>