[Avatar] Remove shape (#10122)

### WHY are these changes introduced?

Fixes #9977

### WHAT is this pull request doing?

- Removes the `shape` prop from `Avatar` as now there is only the
squircle shape
- Cleaned up the `Avatar` stories

.changeset/chilly-grapes-enjoy.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': major
+---
+
+Removed `shape` prop on `Avatar` component

polaris-react/src/components/Avatar/Avatar.scss

@@ -18,6 +18,7 @@
   background: var(--p-color-avatar-background-experimental);
   color: var(--p-color-avatar-color-experimental);
   user-select: none;
+  border-radius: var(--p-border-radius-05);
 
   &.imageHasLoaded {
     background: transparent;
@@ -44,23 +45,6 @@
   }
 }
 
-.shapeRound {
-  border-radius: var(--p-border-radius-05);
-}
-
-.shapeSquare {
-  border-radius: var(--p-border-radius-05);
-
-  .Text {
-    font-size: var(--p-font-size-200);
-    font-weight: var(--p-font-weight-regular);
-  }
-
-  .long {
-    font-size: var(--p-font-size-75);
-  }
-}
-
 .hidden {
   visibility: hidden;
 }