refactor(libutil): remove showBytes() in favor of renderSize()

The `showBytes()` function was redundant with `renderSize()` as the
latter automatically selects the appropriate unit (KiB, MiB, GiB, etc.)
based on the value, whereas `showBytes()` always formatted as MiB
regardless of size.

Co-authored-by: Bernardo Meurer Costa <beme@anthropic.com>
(cherry picked from commit f234633e27)

src/libutil/util.cc

@@ -317,9 +317,4 @@ std::pair<std::string_view, std::string_view> getLine(std::string_view s)
     }
 }
 
-std::string showBytes(uint64_t bytes)
-{
-    return fmt("%.2f MiB", bytes / (1024.0 * 1024.0));
-}
-
 } // namespace nix