Combined Database backend: remove create/delete support

[catsby]

After #6834 was merged we decided to remove the support for creating or deleting database users with the new Static Roles feature. Instead, Vault requires the database user already exist at the time the Static Role is created, and continue to exist for the duration that Vault will manage password rotation. When destroying the Static Role in Vault, we will no longer offer or attempt to also delete the database user. We felt that the core purpose / benefit of this feature is to automate password rotation, and as such we should not cross over the boundary into managing the existence of the database user as well.

This PR removes all database statement fields except rotation_statements, as well as the optional toggle to delete the database user, from the Static Role configuration.

Misc. details:

• refactored out some of the "common" methods like pathRoleReadCommon. Static Roles no longer have any statements other than rotation_statements, so the common methods seemed superfluous.
• refactored tests to always create the database user in advance
• refactored some tests that used to check for database user being created or removed, since Static Roles no longer do that
• refactored the PostgreSQL implementation to only reference the rotation statements

[tyrannosaurus-becks]

Do we need to worry about this test failure?

=== RUN   TestPostgresSQL_SetCredentials
--- FAIL: TestPostgresSQL_SetCredentials (2.61s)
    postgresql_test.go:356: err: empty rotation statements
FAIL plugins/database/postgresql.TestPostgresSQL_SetCredentials (2.61s)
FAIL plugins/database/postgresql

[catsby]

Do we need to worry about this test failure?

@tyrannosaurus-becks Yes, thanks for pointing that out 😄 I fixed that in 1367ae4