Remove tokens package

[kegsay]

GMSL is for federation only.

Usages:

• https://github.com/matrix-org/dendrite/blob/main/clientapi/routing/server_notices.go#L26 - GenerateLoginToken is called and the token passed to userapi.PerformDeviceCreationRequest
• https://github.com/matrix-org/dendrite/blob/main/clientapi/routing/register.go#L40 - same thing but only for guests. Normal users use token, err := auth.GenerateAccessToken(). We should use that instead in all cases.

History:

• This package was added in 2019.
• Intended to be used as macaroons (which are no longer used in Synapse)
• Allows verification of tokens without DB hits.
• In practice we do DB hits and string compare.