crimson/os/alienstore/CmakeLists: Drop WITH_SEASTAR/ALIEN

crimson/os/alienstore/CmakeLists: Drop WITH_SEASTAR/ALIEN

Removing WITH_SEASTAR/WITH_ALIEN will allow us to remove
WITH_ALIEN all together as now "alien" targets
are not defined WITH_SEASTAR.
Mere WITH_SEASTAR is enough to differentiate between "alien"
targets to ones used by Crimson.

Note: The cmake subdirectory of "crimson" will pull WITH_SEASTAR
to crimson-alienstore. That is why "crimson" is replaced by "seastar".
Ideally, we would not need to use "seastar" as well but the pure
alien sources (e.g alien_*.cc) must use seastar's concepts.

This "workaround" results in an crimson-alienstore to not be defined
WITH_SEASTAR even though it uses seastar. While this is not trivial,
it will help with removing WITH_ALIEN which makes Crimson's cmake
harder to understand. Moreover, WITH_SEASTAR trully represents a
Crimson target - and as crimson-alienstore is not trully Crimson,
his "workaround" seems to fit best.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>