panvk: drop fragmentStoresAndAtomics support for now
This is failing a bunch of tests on CI. In particular, it seems some details are wrong WRT robust image access and atomics. There's some lowering in place for that, but a quick test didn't fix it for me, so let's walk this back and try again later.
Fixes: 605c173f ("panvk: update feature support")
/cc @ericsmith