Remove erroneous reference to the base `Symbol` when mapping Rust types (#1439)

`.addReference(this)` adds a reference to the `Symbol` on which
`.mapRustType` was called. This is correct only when `f` is a function
that _wraps_ its input `RustType`; for example, when `f` wraps it in a
`Box` or constructs a `Vec`. However, the code is incorrect for an
arbitrary `f`; for example, when `f` _swaps_ the type.

codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/SymbolVisitor.kt

@@ -101,11 +101,15 @@ fun Symbol.makeOptional(): Symbol {
     }
 }
 
-/** Map the RustType of a symbol with [f] */
+/**
+ * Map the [RustType] of a symbol with [f].
+ *
+ * WARNING: This function does not set any `SymbolReference`s on the returned symbol. You will have to add those
+ * yourself if your logic relies on them.
+ **/
 fun Symbol.mapRustType(f: (RustType) -> RustType): Symbol {
     val newType = f(this.rustType())
     return Symbol.builder().rustType(newType)
-        .addReference(this)
         .name(newType.name)
         .build()
 }