Remove BOARD_SEPOLICY_VERS_DIR build variable.

The original purpose of BOARD_SEPOLICY_VERS_DIR was to allow the
specification of an alternate platform public policy, primarily for
testing purposes.  This should not be a part of the released platform,
since the only public policy and corresponding mapping file construction
should be based on the current public platform policy, with compatibility
with vendor policy targeting previous versions provided by static mapping
files.  Its continued presence muddles the generation of mapping files by
potentially introducing a situation in which an incorrect mapping file is
generated.  Remove it.

Bug: 36783775
Test: Device boots with compiled SELinux policy (SHA256s don't match for
precompiled policy).

Change-Id: I9e2100a7d709c9c0949f4e556229623961291a32