refactor!: do not set primary theme on message-input button (#9680)

Author: web-padawan

packages/message-input/src/vaadin-message-input-mixin.js

@@ -92,8 +92,6 @@ export const MessageInputMixin = (superClass) =>
 
       this._buttonController = new SlotController(this, 'button', 'vaadin-button', {
         initializer: (btn) => {
-          btn.setAttribute('theme', 'primary');
-
           btn.addEventListener('click', () => {
             this.__submit();
           });

packages/message-input/test/dom/__snapshots__/message-input.test.snap.js

@@ -7,7 +7,6 @@ snapshots["vaadin-message-input default"] =
     role="button"
     slot="button"
     tabindex="0"
-    theme="primary"
   >
     Send
   </vaadin-button>
@@ -48,7 +47,6 @@ snapshots["vaadin-message-input disabled"] =
     role="button"
     slot="button"
     tabindex="-1"
-    theme="primary"
   >
     Send
   </vaadin-button>