net: FileListener doesn't work on Plan 9

[mikioh]

See https://storage.googleapis.com/go-build-log/a29b1dce/plan9-386-gcepartial_808d3016.log.

ok      mime/quotedprintable    0.164s
--- FAIL: TestFileListener (0.00s)
    file_test.go:168: file file+net /net/tcp/22/ctl: file does not represent a listener
FAIL

[bcmills]

This issue is very old and the plan9-386-gcepartial builder no longer exists.

[bcmills]

Duplicate of #29801