xdiff: remove "line_hash" field from xrecord_t

xdiff: remove "line_hash" field from xrecord_t

Prior to commit 6a26019c81 (xdiff: split xrecord_t.ha into line_hash
and minimal_perfect_hash, 2025-11-18) the "ha" field of xrecord_t
initially held the "line_hash" value and once the line had been
interned that field was updated to hold the "minimal_perfect_hash". The
"line_hash" is only used to intern the line so there is no point in
storing it after all the input lines have been interned.

Removing the "line_hash" field from xrecord_t and storing it in
xdlclass_t where it is actually used makes it clearer that it is a
temporary value and it should not be used once we're calculated the
"minimal_perfect_hash". This also reduces the size of xrecord_t by 25%
on 64-bit platforms and 40% on 32-bit platforms. While the struct is
small we create one instance per input line so any saving is welcome.

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