libnet/osl: drop netns path GC

[akerouanton]

• relates to moby/libnetwork@05462c2 / Workaround kernel bugs s related to namespaces libnetwork#223
• relates to torvalds/linux@8f502d5 (fix in kernel 4.1)

- What I did

Commit 3ec19ff introduced a GC goroutine to delete files where netns were mounted. It was primarly added to work around a race in kernel 3.18-4.0.1. Since no distros we support are using such old kernels, there's no need to keep this code around.

- How to verify it

Existing tests.

[thaJeztah]

Not related to this PR probably, but I was curious what this once was used for (we should probably rename it to describe what it's for); but looking at that once, the logic seems ... iffy .. it's used to call createBasePath (which is "ok"), but createBasePath depends on basepath(), which returns netnsBasePath, which can be set through an exported function SetBasePath();

• the basepath() function for sure looks redundant (but that's a nit)
• SetBasePath() has no synchronisation
• It also means that SetBasePath can be called after createBasePath ran, and it would not run again (if netnsBasePath was modified

moby/libnetwork/osl/namespace_linux.go

Lines 51 to 68 in 577a795

	// SetBasePath sets the base url prefix for the ns path
	func SetBasePath(path string) {
	netnsBasePath = filepath.Join(path, "netns")
	}
	func basePath() string {
	return netnsBasePath
	}
	func createBasePath() {
	err := os.MkdirAll(basePath(), 0o755)
	if err != nil {
	panic("Could not create net namespace path directory")
	}
	// Start the garbage collection go routine
	go removeUnusedPaths()
	}

[thaJeztah]

I added a link to the original PR in libnetwork (moby/libnetwork#223), which also mentions the fix in the kernel, which went in to kernel 4.1; torvalds/linux@8f502d5

[thaJeztah]

LGTM