diff.c: omit check for line prefix in emit_line_0

As the previous patch made sure we only call emit_line_0 once per line,
we do not need the work around introduced in f7c3b4e2d8 (diff: add an
internal option to dual-color diffs of diffs, 2018-08-13) that would ensure
we'd emit 'diff_line_prefix(o)' just once per line.

By having just one call of emit_line_0 per line, the checks are dead code.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -629,10 +629,7 @@ static void emit_line_0(struct diff_options *o,
 	int nofirst;
 	FILE *file = o->file;
 
-	if (first)
 	fputs(diff_line_prefix(o), file);
-	else if (!len)
-		return;
 
 	if (len == 0) {
 		has_trailing_newline = (first == '\n');