[MAINT] - Remove redundant column "channel" in conda_package_build table

[soapy1]

Context

Follows up #961

Currently, the conda_package_build table has a column for the channel the package comes from (ref: https://github.com/conda-incubator/conda-store/blob/main/conda-store-server/conda_store_server/_internal/orm.py#L786). However, this is redundant information because each entry must also have a conda_package entry associated to it, which already has the channel information (https://github.com/conda-incubator/conda-store/blob/main/conda-store-server/conda_store_server/_internal/orm.py#L743).

This duplication of information can lead to inconsistencies, for example the bug resolved by #961.

Value and/or benefit

This change will simplify the database relations.

Anything else?

No response