Do not use alias_attribute to alias an association

[nvasilevski]

For reference, the code I'm proposing to remove was added in #40835

Using alias_attribute to alias an association is not an indented use case. This commit removes alias_attribute calls to alias an association along with the relevant tests. However, it doesn't mean that the commit brakes any current behaviors.

Currently running Active Record tests result in a bunch of deprecation warnings issued related to alias_attribute :entry, :post definition.
Example:

DEPRECATION WARNING: Comment model aliases `post` and has a method called `post_changed?` defined. In Rails 7.2 `entry_changed?` will not call `post_changed?` anymore.

This is a falsey warning so in order to get rid of it we have two options:

• Fix the code that issues the warning to account for alias_attribute being used to alias an association
• Remove alias_attribute usages to alias an association so the warning goes away with them

Since aliasing an association using alias_attribute is not an intended use case I'm leaning towards removing it from the test suite.

[byroot]

the code I'm proposing to remove was added in #40835

Alright, it was added back because it used to work, but IMO that never was a feature and shouldn't have been fixed in the first place.

People running into this can do manual aliasing, e.g.

def topic
  community_topic
end

def topic=(topic)
  self.community_topic = topic
end

Or contribute an alias_association helper.