diff.c: omit check for line prefix in emit_line_0

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 f7c3b4e2d8c (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>