Simplify branded reg pages to just use brand background img (#1647)

-   Ticket: [ENG-3748]
-   Feature flag: n/a

## Purpose
- Remove some of the complexities around how we use brand colors and images in the registries header

## Summary of Changes
- Remove the logic pertaining to how we show brand color/images on the registries header, so the only thing shown in the header will just be the brand's hero image 
- Accessibility/color contrast on branded registry pages is gonna be a little whacky until we change the brand assets to a more appropriate color

Author: futa-ikeda

lib/registries/addon/components/hero-overlay/styles.scss

@@ -13,7 +13,7 @@
         @include primary-color-bg;
         background: var(--hero-background-img-url);
         background-size: cover;
-        z-index: -1; /* should be in front of the ::before defined below */
+        z-index: -1;
         display: block;
     }
 
@@ -22,20 +22,6 @@
     }
 }
 
-:global(.with-custom-branding) .HeroOverlay {
-    &::before {
-        @include primary-color-bg;
-        display: block;
-        z-index: -2; /* behind the ::after image */
-    }
-
-    &::after {
-        opacity: 0.4;
-        filter: grayscale(1);
-        z-index: -1;
-    }
-}
-
 .AlignCenter {
     align-items: center;
 }