dbfactory: drop the s3-endpoint, s3-region and s3-path-style params

These three params are routing, not authentication: they set BaseEndpoint,
the signing region, and UsePathStyle. Credentials already come entirely
from the AWS SDK chain.

Two problems with carrying them as creation params:

  - Every cloud backend added after aws:// takes no params at all. gs, oci
    and az each define zero and rely on the ambient credential and config
    chain. Reintroducing a param map for s3:// revives the one pattern the
    codebase moved away from.

  - The SDK already resolves endpoint and region ambiently, via
    AWS_ENDPOINT_URL, the per-service AWS_ENDPOINT_URL_S3, the shared
    config endpoint_url key including its per-service form, and AWS_REGION.
    s3-endpoint and s3-region duplicated that surface while covering less
    of it.

Routing will move into the url as query parameters, which earl.Parse
already handles and which is persisted per remote, so a repo can address
two different providers at once without a params map or new flags. Until
that lands, endpoint and region come from the SDK chain and there is no way
to request path-style addressing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>