Reduce the number of extensions

[guillep]

Many extesions are:

• tests extending existing test classes that can be either integrated in the extended class or put in their own class
• extensions that could be integrated in the extended class but maybe were not because of historical reasons
• extensions that are used in a single place, have only that single use case, and can just be inlined in the caller

This PR is a follow up of pharo-project/pharo#18501 that solves many unjustified extensions using the simple heuristics above:

• move to the extended class
• inline in caller
• put in their own class

WARNING ⚠️ Depends on pharo-project/pharo#18501 and should be integrated after it

[estebanlm]

I disagree with the removal of asValueHolder (because the replacement lacks the flexibility of it), but the rest is fine.

[estebanlm]

tag is because I'm waiting for the other side.

[jecisc]

Since this PR is in discussion, I opened another one with a subset of the changes: #1821

The goal of this PR is to just remove the methods overriding methods from the pharo repository

[estebanlm]

so, we should now close this ?

[jecisc]

Some things should still be integrated. But a decision needs to be taken for the #asValueHolder.

I find it easier to us #asValueHolder personally as Esteban