Simplify ECFragGetter by removing code that guards against the policy
fragment_size being None or zero.
Policy fragment_size must be > 0: the fragment_size is based on the
ec_segment_size, which is verified as > 0 when constructing an EC
policy. This is asserted by test_parse_storage_policies in
test.unit.common.test_storage_policy.TestStoragePolicies.
Also, rename client_chunk_size to fragment_size for clarity.
Change-Id: Ie1efaab3bd0510275d534b5c023cb73c98bec90d