xdiff: remove unused data from xdlclass_t

xdiff: remove unused data from xdlclass_t

Prior to commit 6d507bd41a (xdiff: delete fields ha, line, size
in xdlclass_t in favor of an xrecord_t, 2025-09-26) xdlclass_t
carried a copy of all the fields in xrecord_t. That commit embedded
xrecord_t in xdlclass_t to make it easier to change the types of
the fields in xrecord_t. However commit 6a26019c81 (xdiff: split
xrecord_t.ha into line_hash and minimal_perfect_hash, 2025-11-18)
added the "minimal_perfect_hash" field to xrecord_t which is not
used by xdlclass_t. To avoid wasting space stop copying the whole
of xrecord_t and just copy the pointer and length that we need to
intern the line. Together with the previous commit this effectively
reverts 6d507bd41a.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>