Theory_Data extend operation is obsolete and needs to be the identity function

#### ML

`Theory_Data`extend operation is obsolete and needs to be the identity function; merge should be conservative and not reset to the empty value. Subtle INCOMPATIBILITY and change of semantics (due to`Theory.join_theory`from Isabelle2020). Special extend/merge behaviour at the begin of a new theory can be achieved via`Theory.at_begin`.

This refers to Isabelle/efb7fd4a6d1f.

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