changefeedccl: remove changefeed.external_connection.apply_uri_path_and_params.enabled

`changefeed.external_connection.apply_uri_path_and_params.enabled` was added in
#174687 as a temporary escape hatch and should be removed once it is no longer
needed.

### Background

Before #174687, a changefeed sink URI of the form `external:///`
silently discarded ``, so the feed wrote to the external connection's base
URI rather than the requested subdirectory. Query parameters on the `external://`
URI were discarded the same way. The fix now resolves the URI correctly https://github.com/cockroachdb/cockroach/issues/174687.

Because a changefeed re-resolves its stored `external://` URI every time its sink
is built, applying the path moves where an already-running feed writes — on its
next restart, including a rolling upgrade. The setting exists so an operator
surprised by that can put the feed back on the base URI without recreating it. It
is not meant to be a long-lived knob.

### Work

Delete the setting from `pkg/ccl/changefeedccl/changefeedbase/settings.go` and
the branch that reads it in `isqlExternalConnectionProvider.resolve`
(`pkg/ccl/changefeedccl/sink_external_connection.go`), so the path and
parameters are always applied.

Jira issue: CRDB-68433

Start with pkg/ccl/changefeedccl/changefeedbase/settings.go to locate the temporary setting, then inspect isqlExternalConnectionProvider.resolve in pkg/ccl/changefeedccl/sink_external_connection.go and any nearby changefeed tests. Done means the setting and its conditional branch are removed, leaving external URI paths and query parameters always applied.

Written by the indexing model from the issue text.