Remove Naga preference setting from GL and Metal

[kvark]

Follow-up to #3641 Our code paths are more complex that they should be. Duplicating the Naga path both before and after the Cross, depending on the preference, is not needed by anybody. Now that Naga has matured quite a bit, we need to be more aggressive. I believe this PR preserves the old behavior (minus fallback from Cross to Naga), but cleans up the code. It also avoids unnecessary work (naga parsing of SPIR-V) if Cross is enabled. PR checklist:

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

[kvark]

bors r+

[bors[bot]]

Build succeeded:

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