hook_entity_type_alter: Ability to remove a link template

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New function should be typehinted.
Will need test coverage
When solution is agreed upon it should be added to the issue summary.
Change record will be needed as well.

I fell into this issue when trying to override another entity type's links definition due to the fact that without it I couldn't use the "uri_callback" property.

I have created a MR with the changes mortim07 added. I also checked Drupal core code and it seems that "remove" is more commonly used than "unset" (like Niklan proposed) or even "delete". For example, we have "removeDefinition", "removeListener", "removeSubscriber" etc. So "removeLinkTemplate" is the way to go.

I have also updated the issue summary. So, if there are no objections, the only thing missing is the writing of the tests.

Adding some related issues I found which may guide us on whether we really do want this functionality or another workaround should be found to achieve what we want in general.