pkg/system: remove Lgetxattr, Lsetxattr stubs for non-Linux

It's only used in Linux code. Remove the stubs, which are not carried
forward into the module.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/system/xattrs_unsupported.go

@@ -1,13 +0,0 @@
-//go:build !linux
-
-package system
-
-// Lgetxattr is not supported on platforms other than linux.
-func Lgetxattr(path string, attr string) ([]byte, error) {
-	return nil, ErrNotSupportedPlatform
-}
-
-// Lsetxattr is not supported on platforms other than linux.
-func Lsetxattr(path string, attr string, data []byte, flags int) error {
-	return ErrNotSupportedPlatform
-}