Auto merge of #18753 - upsuper:binding-structs, r=emilio

Only generate structs file for the current build

This PR changes build_gecko.rs to only generate the `structs.rs` for the current build (rather than both), depending on whether `gecko_debug` feature is set.

The in-tree files are switched to use the previous release one, because that's what we currently use for stylo test and it is in general what we really care about. For this change, `gecko_debug` mode is removed from Servo CI (in `build-geckolib`) with the assumption that people general do that build locally for stylo development, so it is less likely to be broken than release.

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python/servo/build_commands.py

@@ -442,8 +442,6 @@ class MachCommands(CommandBase):
             opts += ["-v"]
         if release:
             opts += ["--release"]
-        else:
-            features += ["gecko_debug"]
 
         if features:
             opts += ["--features", ' '.join(features)]