Drop the support for BSON

[bjsowa]

@EzraBrooks Following the conversation from RobotWebTools/roslibjs#1039. There's an idea of dropping support for BSON encoding to reduce complexity and make rosbridge easier to maintain.

There are a few more arguments for this change:

• There are no tests for bson encoding in either rosbridge server or roslibjs (it might not even work) and other client libraries don't even support it (ros-sharp dropped the support last year).
• Haven't actually confirmed it but it seems that BSON encoding does not give advantage over CBOR when it comes to message size.
• BSON is implemented as a top-level encoder, setting bson_only_mode parameter enforces BSON encoding for all messages, as opposed to CBOR, which can be set per subscription/service. There's no way for the client to query whether the rosbridge server will use BSON for serialization.

[bjsowa]

@sea-bass @MatthijsBurgh