Auto merge of #16121 - froydnj:unused-property-mako-imports, r=cbrewster

remove unused `use` statements in mako helpers

These are no longer needed, according to `./mach build -d` and grep.

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because compilation is sufficient

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components/style/properties/helpers.mako.rs

@@ -219,8 +219,6 @@
         use properties::{DeclaredValue, LonghandId, LonghandIdSet};
         use properties::{CSSWideKeyword, ComputedValues, PropertyDeclaration};
         use properties::style_structs;
-        use std::boxed::Box as StdBox;
-        use std::collections::HashMap;
         use std::sync::Arc;
         use values::computed::{Context, ToComputedValue};
         use values::{computed, specified};