Drop asn field from dcim.Site

[jeremystretch]

Proposed Changes

Remove the asn field from the dcim.Site model.

Justification

A new ASN model was introduced in v3.1 (#6732), to which the Site model has a many-to-many relationship. The static value field is no longer needed.

[DanSheps]

Should we deprecate the filter asn= and asn_id= or leave that and have those query against the ASN model instead?