remove 'genome model reference-sequence generate-bitmask'

This command calls reallocate on
Genome::Model::Build::ImportedReferenceSequence which it cannot due so
this command, if used, would crash.  There does not seem to be a record
of its usage in the usage_log.

CHANGELOG: Remove 'genome model reference-sequence generate-bitmask',
which appears to be unused and broken.
Merge pull request #15 from nnutter/master

remove 'genome model reference-sequence generate-bitmask'