Remove obsolete mapping build rules.

Mapping files for previous releases are unconditionally packaged on the
device. No need to account for case when BOARD_SEPOLICY_VERS and
PLATFORM_SEPOLICY_VERSION are different.

Bug: 119305624
Test: m selinux_policy
Change-Id: I36c3c43f96870d9a71adf91c8fb8926587c5a50e