lsm: drop LSM_ID_IMA

When IMA becomes a proper LSM we will reintroduce an appropriate
LSM ID, but drop it from the userspace API for now in an effort
to put an end to debates around the naming of the LSM ID macro.

Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>