broadcom/compiler: remove v3d_fs_key depth_enabled field.

It is not used right now, so keeping it adds some noise/confusion.

So far configuring Z test are done through the CFG_BITS. See v3dX(emit_state) at v3dx_emit.c for v3d, and pack_cfg_bits at v3dv_pipeline.c for v3dv. There flags like z_updates_enable and others are filled up.

That key field seems like a leftover coming from using vc4 as reference, as that driver defines and uses a field with name name.