polish(payments-api): Remove root accessor and default api path prefix

[david1alvarez]

Because:

• Payments API shares code and services with other applications in the fxa ecosystem, but came with a default /api/ base path prefix for all routes
• This prefix causes some confusion for the service health checks, which expect all heartbeat and version endpoints to be off of the root path for all services

This commit:

• Removes the /api/ base prefix, as it is not required by internal or external stakeholders, and appears to be an inherited default value
• removes the root ('/') endpoint, as its purpose is not yet defined

Closes #N/A (relates to SVCSE-4181)

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