x/pkgsite: remove double check for doc escape rendering

[jba]

To get our tests to pass on CI, which includes tip and the last two versions of Go, we must check for two variants of rendered doc: those with smart quotes and those without.

Once all Go versions have the change removing smart quotes, we can test just one version.

The relevant code is in internal/godoc/dochtml/internal/render/linkify_test.go, function TestFormatDocHTML.

[gopherbot]

Change https://go.dev/cl/801520 mentions this issue: /internal/godoc/dochtml/internal/render: fix tests

[prattmic]

This may be related to #80429.