fix(opentelemetry): remove plugin attr set_ngx_var

[Baoyuantop]

Description

We changed the attribute of the Opentelemetry plugin to metadata in PR #11940. After that, users can set the set_ngx_var property in metadata. However, the generated template file still relies on the set_ngx_var property from the plugin attribute to predefine variables https://github.com/apache/apisix/blob/master/apisix/cli/ops.lua#L553-L556, which will confuse users. This PR removes this judgment and allows this property to be controlled in metadata only.

Which issue(s) this PR fixes:

Fixes #12402

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)