test/kubernetes: use zero-value TextParser until prometheus/common is bumped

[SAY-5]

What this PR does / why we need it:

expfmt.NewTextParser was introduced in a later prometheus/common release than the one pinned here (v0.59.1), so every CoreDNS PR that runs the Kubernetes test step fails with:

./metrics_test.go:87:15: undefined: expfmt.NewTextParser

(reported against coredns/coredns#7731 - e.g. the Circle-CI failure on coredns/coredns#7728).

Revert that call site to the zero-value var tp expfmt.TextParser pattern (which defaults to legacy validation) so the job compiles on the currently pinned dependency. Once go.mod's prometheus/common is bumped to a version that exports NewTextParser, this can be re-applied without the compat note.

Also drops the now-unused prometheus/common/model import.

Release note:

NONE

[SAY-5]

The five kubernetes-tests-v1.25..1.29 CircleCI jobs have been red since the PR opened in late April; same red across every K8s version, which suggests the CircleCI runner image / kind setup is broken for these branches independently of this small text-parser revert. The change only touches test/kubernetes/metrics_test.go and stops calling expfmt.NewTextParser(model.LegacyValidation) (which doesn't exist in the pinned prometheus/common v0.59.1) so the parse compiles again. Could a maintainer retrigger CI or look at whether the kind images need refreshing?