Fix external container-based tests JDK_VERSION setting

In several external test `test.sh` file, we have the command to unset `JDK_VERSION` and use auto-detect ( [code link](https://github.com/adoptium/aqa-tests/blob/2a4cdaf7678919a844d35ef9d8a9b4ad810cad9e/external/functional-test/test.sh#L26-L27) ), this was because this parameters were not passed correctly. We can further improve it by correcting the passed `JDK_VERSION` parameter.
- In [`external.sh`](https://github.com/adoptium/aqa-tests/blob/2a4cdaf7678919a844d35ef9d8a9b4ad810cad9e/external/external.sh#L320) and [`build_image.sh`](https://github.com/adoptium/aqa-tests/blob/2a4cdaf7678919a844d35ef9d8a9b4ad810cad9e/external/build_image.sh#L92) files (maybe also other files), check where this parameter was not passed correctly, fix it.
- Then for all external tests, remove `JDK_VERSION` unset command as it will not be needed.

Inspect external/external.sh and external/build_image.sh at the mentioned JDK_VERSION handling, then compare them with external/functional-test/test.sh and the other external test.sh files. Verify where JDK_VERSION is passed incorrectly, correct those usages, and remove the unset commands from all external tests; done means the parameter is passed consistently and auto-detection is no longer forced.

Written by the indexing model from the issue text.