libdnssec: remove PKCS #8 keystore abstraction

Knot.files

src/knot/dnssec/kasp/keystore.c

src/libdnssec/Makefile.inc

src/libdnssec/keystore.h

src/libdnssec/keystore/internal.h

src/libdnssec/keystore/keystore.c

src/libdnssec/keystore/pkcs11.c

src/libdnssec/keystore/pkcs8.c

src/libdnssec/keystore/pkcs8_dir.c

tests/Makefile.am

tests/libdnssec/test_keystore_pkcs8.c

tests/libdnssec/test_keystore_pkcs8_dir.c