Remove `counter` load generator source from the Terraform provider

[bobbyiliev]

Feature request

As per this PR here: MaterializeInc/materialize#33847, The counter load generator source will be removed from Materialize in a future release.

We should update the Materialize Terraform provider to remove support for the counter source type. Any existing examples, definitions, tests, or documentation referencing it should be cleaned up as part of this change.

Acceptance Criteria:

• Remove the counter load generator source from the provider schema.
• Update or delete any related examples and documentation.
• Confirm that provider tests no longer reference the counter source.
• Update all affected tests