pv-runtime: Don't try to mask /etc/vulkan/*.d
Since updating our Flatpak-derived code, every time we start a container we log messages like:
N: Not replacing "/etc/vulkan/implicit_layer.d" with tmpfs: Path "/etc" is reserved by the container framework
We only import selected directories from /etc into the container's /etc, and none of the directories below /etc/vulkan are among them, so there is actually no need to mask these directories. Silence the warning by not attempting to mask them.