fix(pgproto): remove use_tarantool_parameter_placeholders

Summary

• fix(pgproto): remove use_tarantool_parameter_placeholders

We don't need to replace $n with question marks any more, as we support the pg parameters format.

See sbroad!377 (merged) and sbroad#543 (closed).

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• Close https://git.picodata.io/picodata/picodata/pgproto/-/issues/18
• Cherry-pick to: none
• Docs follow-up: not necessary / new issue