remove credential offer part from credential profiles

[tlodderstedt]

After the recent changes, there is no longer any format specific part in a credential offer. This PR removes the respective definitions from the credential format profiles.

[Sakurann]

When paul did a previous PR, I suggested not to remove entirely to show concrete examples how to use credential format specific identifiers, etc..
#109 (review)

[paulbastian]

I don't have strong opinions on this

I think having more than one example is nice and the credential offer in appendix could match to the credential issuer metadata. Especially having some examples for the changes in #95 would be nice.

On the other hand, we don't have examples on e.g. authroization response either and we could move some examples to the credential offer section if we want more examples

[Sakurann]

I came along with this.
but as @jogu said during the WG call, would like to see the example files unused elsewhere in the spec removed before we merge this.
also it does not compile as-is..

[paulbastian]

I think it would be good to include at least one of the existing examples into the credential offer section, to see some variations of transaction code

[paulbastian]

I did some changes, please review
@tlodderstedt
Also closes #114