Remove distribution methods from BridgeDistribution option

From the tor manpage:

       BridgeDistribution string
           If set along with BridgeRelay, Tor will include a new line in its
           bridge descriptor which indicates to the BridgeDB service how it
           would like its bridge address to be given out. Set it to "none" if
           you want BridgeDB to avoid distributing your bridge address, or
           "any" to let BridgeDB decide. (Default: any)

We currently allow bridge operators to choose which distribution method their bridge is handed out over. We're re-evaluating the usefulness of this option compared to the complexity required to implement it in our work on tpo/anti-censorship/rdsys#12 (closed).

The addition of this config option was first proposed in #18329 (closed) for the purpose of distinguishing between bridges that were to be handed out by BridgeDB and bridges that wished not to be handed out by BridgeDB so that we could still collect metrics from its bridge descriptor. The use case then got expanded to allow operators choice of any existing distribution method, but it's not clear to me why a bridge operator would want a choice beyond the original "do or do not distribute my bridge for me". Or perhaps more relevant: how this choice would help the Tor network. The main reason I can think of is experiments or testing purposes for researchers to measure enumeration by spinning up a bridge for a specific distribution method.

I'm happy to implement this feature if we decide we have good reasons for it. However, removing it would simplify our implementation of rdsys.