fast-import: remove dead strbuf

fast-import: remove dead strbuf

The strbuf in `note_change_n` is to copy the remainder of `p` before
potentially invalidating it when reading the next line. However, `p` is
not used after that point. It has been unused since the function was
created in a8dd2e7d2b (fast-import: Add support for importing commit
notes, 2009-10-09) and looks to be a fossil from adapting
`file_change_m`. Remove it.

Signed-off-by: Thalia Archibald <thalia@archibald.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>