expvar: make possible to delete all exported variables

[artemyarulin]

By default expvar publishes cmdline and memstats which is not possible to remove.
This PR makes it possible by moving existing function RemoveAll from tests

Fixes #29105

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Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

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[ryandotsmith]

I'd love to see this, or something like this, land. memstats is harshing my mellow and there is literally no way of removing it while also using expvar.