Remove some asset examples from web showcase (#14973)

# Objective

- `custom_asset_reader` and `extra_asset_source` examples are not
working on web.
- Fixes #14689

## Solution

- Make these examples `wasm=false` per
#14689 (comment)

## Testing

Cargo.toml

@@ -1407,7 +1407,8 @@ doc-scrape-examples = true
 name = "Custom Asset IO"
 description = "Implements a custom AssetReader"
 category = "Assets"
-wasm = true
+# Incompatible with the asset path patching of the example-showcase tool
+wasm = false
 
 [[example]]
 name = "embedded_asset"
@@ -1429,7 +1430,8 @@ doc-scrape-examples = true
 name = "Extra asset source"
 description = "Load an asset from a non-standard asset source"
 category = "Assets"
-wasm = true
+# Uses non-standard asset path
+wasm = false
 
 [[example]]
 name = "hot_asset_reloading"