Remove removeTypenameFromVariableExchange to be agnostic to GQL client

[IzumiSy]

Currently, fabrix internally depends on urql, but we have a plan to make it pluggable by #123

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Background

Since fabrix automatically adds __typename in its selection that is needed to look up the field type to decide the component to render, in the case of executing update mutation directly with the entities fetched from another queries, __typename should manually be removed before sending the mutation requests to the server without the custom exchange of urql (removeTypenameFromVariableExchange) to avoid an error caused by unmatched fields.

https://github.com/fabrix-framework/fabrix/blob/main/packages/fabrix/src/exchanges/removeTypenameFromVariable.ts

However, the behaviour does not have to be implemented as an urql exchange, so remove removeTypenameFromVariableExchange and implement the behaviour as just a simple object manipulation instead.