refactor: Drop util::Result operator=

`util::Result` objects are aggregates that can hold multiple fields with
different information. Currently Result objects can only hold a success value
of an arbitrary type or a single bilingual_str error message. In followup PR
https://github.com/bitcoin/bitcoin/pull/25722, Result objects may be able to
hold both success and failure values of different types, plus error and warning
messages.

Having a Result::operator= assignment operator that completely erases all
existing Result information before assigning new information is potentially
dangerous in this case. For example, code that looks like it is assigning a
warning value could erase previously-assigned success or failure values.
Conversely, code that looks like it is just assigning a success or failure
value could erase previously assigned error and warning messages.

To prevent potential bugs like this, disable Result::operator= assignment
operator.

It is possible in the future we may want to re-enable operator= in limited
cases (such as when implicit conversions are not used) or add a Replace() or
Reset() method that mimicks default operator= behavior. Followup PR
https://github.com/bitcoin/bitcoin/pull/25722 also adds a Result::Update()
method providing another way to update an existing Result object.

Co-authored-by: stickies-v <stickies-v@protonmail.com>

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