Remove support for the pipeline service account

[qu1queee]

Idea:

When users do not specify a service account to use, we have a mechanism for defaulting into one of two service accounts.

First we try to see if we have a sa in the namespace of the name pipeline, if thats not the case, we try to use the default one.

This issue is for removing the logic around the default one, as stated in https://github.com/shipwright-io/build/issues/662