ocivm: Do not mount tmpfs for Portage

Very large packages cannot be built with `PORTAGE_TMPDIR` on a tmpfs
mount, unless the VM has a tremendous amount of RAM.  The performance
improvement is negligible anyway, so let's not bother.

ocivm.sh

@@ -17,7 +17,5 @@ ocivm \
     -v "${PWD}":/tmp/src \
     -v portagecache:/var/cache:size=16G \
     -v portage:/var/db/repos/gentoo:size=4G \
-    --tmpfs /usr/aarch64-unknown-linux-gnu/tmp \
-    --tmpfs /var/tmp \
     localhost/cross-aarch64-unknown-linux-gnu \
     "$@"