fix(gaussdb/proxy): unset all parameters because no response

[Lance52259]

What this PR does / why we need it: unset all parameters of mysql proxy resource because there is no response in the query API.

Which issue this PR fixes: (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged) fixes #xxx

Special notes for your reviewer:

Release note:

1. unset all parameters because no response.

PR Checklist

• [x] Tests added/passed.
• [ ] Documentation updated.
• [ ] Schema updated.

Acceptance Steps Performed

make testacc TEST='./huaweicloud/services/acceptance/gaussdb' TESTARGS='-run=TestAccGaussDBProxy_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/gaussdb -v -run=TestAccGaussDBProxy_basic -timeout 360m -parallel 4
=== RUN   TestAccGaussDBProxy_basic
=== PAUSE TestAccGaussDBProxy_basic
=== CONT  TestAccGaussDBProxy_basic
--- PASS: TestAccGaussDBProxy_basic (1497.07s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/gaussdb   1497.151s