sepolicy: Drop BOARD_SEPOLICY_IGNORE/REPLACE support.

With changes I431c1ab22fc53749f623937154b9ec43469d9645 and
Ia54aa263f2245c7090f4b9d9703130c19f11bd28, it is no longer
legitimate to use BOARD_SEPOLICY_IGNORE or REPLACE with
any of the *_contexts files since the CTS requires the AOSP
entries to be present in the device files.

Further, these changes render BOARD_SEPOLICY_IGNORE unusable for
most policy files since all domains and types referenced within any
of the AOSP *_contexts entries must be defined in the kernel policy, so
you cannot use BOARD_SEPOLICY_IGNORE to exclude any .te file
that defines a type referenced in any of those *_contexts files.
There does not seem to be a significant need for such a facility,
as AOSP policy is small and only domains and types used by most
devices should be defined in external/sepolicy.

BOARD_SEPOLICY_REPLACE is commonly misused to eliminate neverallow rules
from AOSP policy, which will only lead to CTS failures, especially
since change Iefe508df265f62efa92f8eb74fc65542d39e3e74 introduced neverallow
checking on the entire policy via sepolicy-analyze.  The only remaining
legitimate function of BOARD_SEPOLICY_REPLACE is to support overriding
AOSP .te files with more restrictive rule sets.  However, the need for this
facility has been significantly reduced by the fact that AOSP policy
is now fully confined + enforcing for all domains, and further restrictions
beyond AOSP carry a compatibility risk.

Builders of custom policies and custom ROMs still have the freedom to
apply patches on top of external/sepolicy to tighten rule sets (which are
likely more maintainable than maintaining a completely separate copy of
the file via BOARD_SEPOLICY_REPLACE) and/or of using their own separate
policy build system as exemplified by
https://bitbucket.org/quarksecurity/build-policies



Change-Id: I2611e983f7cbfa15f9d45ec3ea301e94132b06fa
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>