Obsolete BOARD_PLAT_*_SEPOLICY_DIR

These were marked as deprecated in late 2020 [1]. Shortly later, there
was an attempt to obsolete them [2] for products shipping with API level
31 or greater, but that was reverted [3] because of internal users.

1: I58c64839cc513ae082cd3ee3c1e108843ea7439e
2: Ic4d1164be611836f6aa697fbf1cb1f1c73a3cd39
3: I7d3556aa0f06684b43f80f09e4c8194c6c44336c

Since these are causing warnings on every build, and there are no more
internal users, just obsolete them entirely. We don't need to rely on
the product shipping api level, since that does not guarantee
source-level compatibility -- and this is an easy fix even if people
have been ignoring the real warnings.

Bug: 235414673
Test: treehugger
Change-Id: If803a33efc38a970247919bf224c12b8c717f955
(cherry picked from commit b34c3874)