fix: Remove assertion on HttpApiKey scheme being present in client co… (#4309)

## Description
- Removes erroneous assertion, further explained in
#4226
- Closes #4226


## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: Aaron Todd <aajtodd@users.noreply.github.com>
Co-authored-by: Landon James <lnj@amazon.com>

Author: 3 people

.changelog/1758067735.md

@@ -0,0 +1,9 @@
+---
+applies_to: ["client"]
+authors: ["haydenbaker"]
+references: ["smithy-rs#4226"]
+breaking: false
+new_feature: false
+bug_fix: true
+---
+Fixed problematic assertion on HttpApiKeyAuthTrait `scheme`, which was causing client-codegen to fail when the correct settings for api-key based auth were set.

codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/customizations/HttpAuthDecorator.kt

@@ -180,13 +180,7 @@ private class HttpAuthServiceRuntimePluginCustomization(
                         val trait = serviceShape.getTrait<HttpApiKeyAuthTrait>()!!
                         val location =
                             when (trait.`in`!!) {
-                                HttpApiKeyAuthTrait.Location.HEADER -> {
-                                    check(trait.scheme.isPresent) {
-                                        "A scheme is required for `@httpApiKey` when `in` is set to `header`"
-                                    }
-                                    "Header"
-                                }
-
+                                HttpApiKeyAuthTrait.Location.HEADER -> "Header"
                                 HttpApiKeyAuthTrait.Location.QUERY -> "Query"
                             }