Unload the extra_decls contents in the interpreter mode after interface generation (#24475)

When -fwrite-if-simplified-core (implied by -fbyte-code-and-object-code or -fprefer-byte-code) is on, IfaceTopBind is populated in extra_decls for serializing the Core expressions into the interface files. After the serialization, the contents are not needed any more in the interpreter mode since byte-code objects are already created from the interpretation. To save memory, it is desirable to unload it.