Remove the ClearCookies and ClearCache methods from ContentBrowserClient
These methods were used by content/ to request the deletion of cookies
and cache from embedder. This should no longer be necessary, since
BrowsingDataRemover has been moved to content/ and can be used directly.
Embedders that need to execute additional deletions can still subclass
BrowsingDataRemoverDelegate and implement them there.
There were only two ContentBrowserClient subclasses that used
1. ChromeContentBrowserClient::ClearCache() was used by dev_tools/.
This was replaced by an equivalent call to BrowsingDataRemover.
2. In android_webview, ClearCookies() and ClearCache() were defined,
but not called. ClearCache() used android_webview::RemoveHttpDiskCache
for cache deletion, which is in fact a subset of functionality that
already exists in BrowsingDataRemover, and can be replaced by it.