Remove redundant privateca cert authority tests

[modular-magician]

Partially addresses #12138 (but not sure it's gonna be quite enough)

There's fairly thorough coverage of state management in privateca's ca resource, which is nice-to-have but the global limit on the number of CAs (see #12198 for some discussion around similar limits) and the 30d deletion period mean reducing the total count we provision is valuable. Here's why I cut each of these:

• TestAccPrivatecaCertificateAuthority_rootCaIsEnabledByDefault appears to just be a case where we used a basic config and checked a value, so move the check to the start of TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthorityUpdate (which starts in the same place)
• TestAccPrivatecaCertificateAuthority_rootCaCreatedInStaged is redundant with the first step of TestAccPrivatecaCertificateAuthority_rootCaManageDesiredState
• TestAccPrivatecaCertificateAuthority_subordinateCaCreatedInAwaitingUserActivation , TestAccPrivatecaCertificateAuthority_subordinateCaActivatedByFirstPartyIssuerOnCreation, are each checking the value of an output-only field as returned by the API

We can likely bootstrap the CA used for the datasource and cert tests as well, which would further remove 4 CAs/run.

Oh, also remove privateca_quickstart from the CA pool docs- it doesn't have tests, and it's really a configuration we'd include against the cert resource normally.

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Derived from GoogleCloudPlatform/magic-modules#6338

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