net: drop flakey TestDialFailPDLeak
net: drop flakey TestDialFailPDLeak

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 issue 6553.

LGTM=josharian, iant
R=iant, josharian
CC=golang-codereviews
https://codereview.appspot.com/98080043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang