Cleanup Empty backend, remove AcquireMode from Metal

[kvark]

Just a maintenance PR to clean up some code. PR checklist:

• [x] make succeeds (on *nix)
• [ ] make reftests succeeds
• [ ] tested examples with the following backends:

[kvark]

bors r+

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable