v4: Remove or implement dot1q_mode_from_vlans()

[jtdub]

Description

dot1q_mode_from_vlans() is an abstract method on HConfigViewBase that is not implemented on any platform — all 5 view implementations raise NotImplementedError. Tests exist only to confirm it raises the error.

Proposed Change

Either:

1. Implement it on Cisco IOS and HP ProCurve (which have full VLAN support) as a reference, or
2. Remove it from the abstract base and add it as an optional method on platforms that can support it

Current State

Cisco IOS:    NotImplementedError
Cisco NXOS:   NotImplementedError
Cisco XR:     NotImplementedError
Arista EOS:   NotImplementedError
HP ProCurve:  NotImplementedError