notmuch show: Don't show the subject line twice.

I recently added a print of the subject line for use as part of a
two-line summary in the emacs client. But of course, the subject was
already being printed on the next line. So I didn't really need to add
anything, I could have just stopped hiding what was already
printed. Anyway, we now avoid printing it twice in a row.

notmuch.c

@@ -1076,7 +1076,7 @@ show_command (void *ctx, unused (int argc), unused (char *argv[]))
     int ret = 0;
 
     const char *headers[] = {
-	"Subject", "From", "To", "Cc", "Bcc", "Date"
+	"From", "To", "Cc", "Bcc", "Date"
     };
     const char *name, *value;
     unsigned int i;