fix(header): remove Shadow from Ionic Header (#30818)

Issue number: internal

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## What is the new behavior?
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- The header should not have a shadow.
- Following the design experience, this shadow shouldn't exist. If
needed, the dev should use the divider prop

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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Co-authored-by: ionitron <hi@ionicframework.com>

core/src/components/header/header.ionic.scss

@@ -5,7 +5,6 @@
 // --------------------------------------------------
 
 ion-header {
-  box-shadow: #{globals.$ion-elevation-2};
   z-index: 10; // TODO(ROU-10853): replace this value with a layer token.
 
   &.header-divider {