Remove reference to EF-internal TpcTablesExpression

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In #2366, I added NpgsqlSetOperationTypeResolutionCompensatingExpressionVisitor, which adds an explicit convert node on the leftmost set operation operand, to work around problematic PG type inference behavior.

At the moment, this visitor needs to visit the EF-internal TpcTablesExpression. @smitpatel will be looking at changes on the EF side to obviate this. This is non-critical.

[smitpatel]

I am already working on this. Many edge cases but it will be done in 7.0

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Thanks @smitpatel, no rush as I said, I have no problem even shipping like this.

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This was done.