TestDialFailPDLeak was created for testing runtime-integrated netwrok
poller stuff and used during Go 1.2 development cycle. Unfortunately
it's still flakey because it depends on MemStats of runtime, not
pollcache directly, and MemStats accounts and revises its own stats
occasionally.

For now the codepaths related to runtime-intergrated network poller
are pretty stable, so removing this test case never suffers us.

Fixes #6553.

LGTM=josharian, iant
R=iant, josharian
CC=golang-codereviews
https://golang.org/cl/98080043