golang templates: reduce dbtx interface when not using prepared queries

[robertoamici]

Hi there,

i havent seen any PR requirements or existing tests on the repo so this is just the bare bone change.

Basically I am conditionally making the DBTX interface in the golang sql.DB implementation a little smaller by removing the PrepareContext when prepared queries are turned off, since all usages of PrepareContext are in the part of the template behind the same if statement. In general, if a method is not used, why add it to the interface?

For some context, we ran across this because we are trying to pass to sqlc a wrapper of sql.DB (from https://pkg.go.dev/github.com/honeycombio/beeline-go/wrappers/hnysql) which has a slightly different PrepareContext return type. We don't use prepared queries so that difference shouldn't impact us.

[kyleconroy]

The expected test output needs to be updated using make regenerate.