fix outdated command "rebuild_cache"

[erikyao]

MyVariantHubServer::rebuild_cache method, defined in src/bin/hub.py#L67, uses a get_latest_build(build_name) function. However this function was removed in ecb98dd.

What's more, the build_name parameter seems to be a build_config name actually (e.g. "warm_hg19"). A new signature design is to remove the parameter build_name at all, with target being the target build name (e.g. "warm_hg19_20220127_fpgrwjzd"). The calling of self.managers["build_manager"].list_sources(build_name) can be replaced by reading src_build.<_id>.sources field.

[erikyao]

Decision after 11/01 BioThings Technical meetup: comment out the function and command definition temporarily.