oci: appendOSMounts(): remove unused error, and move

This function was added in ae22854e2b, but never
returned an error, and the error-return was not handled on the callsite. This
patch removes the unused error return, and moves it to a file related to mounts,
which allowed for some of the stubs to be removed and shared between non-FreeBSD
platforms.

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

oci/mounts.go

@@ -68,3 +68,6 @@ func defaultMounts() []specs.Mount {
 		},
 	}
 }
+
+// appendOSMounts is only used on FreeBSD, and a no-op on other platforms.
+func appendOSMounts(_ *Spec, _ string) {}