Remove get_columns implementation for column metadata generation

[mikealfare]

resolves #914

Problem

We implemented redshift-connector's get_columns method for column metadata generation behind a behavior flag. However, it appears there are scenarios where get_columns does not find an object whereas the existing custom query does find this object. This is causing failures when this new functionality is turned on. When the functionality is turned off, the user experiences a warning every run.

Solution

We have elected to remove the new approach for now as turning it on is not an option and turning it off results in a warning on which the user cannot take action. The new methods and the behavior flag have all been removed. The relevant tests have been skipped so that they may be used again once the source of the discrepancy has been resolved.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

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