opensearch: increase test coverage

[Jakob3xD]

Description

Depends on https://github.com/opensearch-project/opensearch-go/pull/512 to be merged first.

• removed unused structs and functions from opensearch
• adjust opensearch test to increase coverage
• fix opensearchtransport to check for empty response body

Issues Resolved

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.24%. Comparing base (06a6dc8) to head (8a15689). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #517 +/- ## ========================================== + Coverage 57.29% 63.24% +5.94% ========================================== Files 315 315 Lines 9823 7715 -2108 ========================================== - Hits 5628 4879 -749 + Misses 2902 1550 -1352 + Partials 1293 1286 -7 ``` | [Flag](https://app.codecov.io/gh/opensearch-project/opensearch-go/pull/517/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/opensearch-project/opensearch-go/pull/517/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `55.78% <0.00%> (+4.94%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/opensearch-project/opensearch-go/pull/517/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `81.45% <100.00%> (+68.60%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/opensearch-project/opensearch-go/pull/517?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | Coverage Δ | | |---|---|---| | [opensearch.go](https://app.codecov.io/gh/opensearch-project/opensearch-go/pull/517?src=pr&el=tree&filepath=opensearch.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project#diff-b3BlbnNlYXJjaC5nbw==) | `92.72% <100.00%> (+17.10%)` | :arrow_up: | | [opensearchtransport/discovery.go](https://app.codecov.io/gh/opensearch-project/opensearch-go/pull/517?src=pr&el=tree&filepath=opensearchtransport%2Fdiscovery.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project#diff-b3BlbnNlYXJjaHRyYW5zcG9ydC9kaXNjb3ZlcnkuZ28=) | `64.70% <100.00%> (+1.62%)` | :arrow_up: | ... and [248 files with indirect coverage changes](https://app.codecov.io/gh/opensearch-project/opensearch-go/pull/517/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project)