Remove BOARD_KERNEL_CONFIG_FILE pacakging logic

This board config value is not used by any products in the tree
(determined using codesearch).
To simplify soong migration, remove this unused variable from make
packaging.

Test: presubmits
Bug: 399788685
Change-Id: Iba95e4572b8fbdc0885675c4b9d7cbdea8da49eb