Remove the last allow-listed unstable features

This CL removes the custom_inner_attributes and mixed_integer_ops
features from the allow-list.  This also allows us to remove the `-A
stable-features` flag.

Bug: 267698452
Test: m rust
Change-Id: I4770c2e9477337f112732c354bf477a99d11a467
diff --git a/rust/builder.go b/rust/builder.go
index 0aa2225..0dfaef4 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -261,7 +261,7 @@
 	// Disallow experimental features
 	modulePath := android.PathForModuleSrc(ctx).String()
 	if !(android.IsThirdPartyPath(modulePath) || strings.HasPrefix(modulePath, "prebuilts")) {
-		rustcFlags = append(rustcFlags, "-Zallow-features=\"custom_inner_attributes,mixed_integer_ops\"")
+		rustcFlags = append(rustcFlags, "-Zallow-features=\"\"")
 	}
 
 	// Collect linker flags