Don't store an operation vector for the decoder

The operation vector is fast but the number of combinations (hole-locations and
repair-symbols) result in to many cache misses for all but the smallest block
sizes.
Testing with randomized hole-locations and repair-symbols show that caching the
matrix created in the gen-step is faster for a block-size of 26 and up. Another
benefit is that the key-size is smaller.

Signed-off-by: Anders Martinsson <anders.martinsson@intinor.se>