loci: remove minimum wire buffer allocation size

This hasn't been useful since writable wire buffers were changed to be
allocated at maximum size. This code was causing excessive memory usage in
Indigo when storing the instruction list, which is often only around 16 bytes.
Removing this code saves us 17% memory usage and increases flow-add performance
by 9%.
Merge into master from pull request #286:
loci: remove minimum wire buffer allocation size (https://github.com/floodlight/loxigen/pull/286)