runtime/pprof: assert that labelHog samples are always labeled
runtime/pprof: assert that labelHog samples are always labeled

With https://golang.org/issue/50007 resolved, there are no known issues
with pprof labels remaining. Thus, the 10% allowed error in
TestLabelSystemstack should not be required.

Drop it in favor of an explicit assertion that all samples containing
labelHog are properly labeled.

This is no flaky in my local testing. It is possible that other bugs
will appear at larger testing scale, in which case this CL will be
reverted, but then at least we will be aware of additional failure
modes.

For #50007.

Change-Id: I1ef530c303bd9a01af649b8b08d4b35505e8aada
Reviewed-on: https://go-review.googlesource.com/c/go/+/369744
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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