Use new cache bin naming in hook_flush_cache

Since the temporary backward compatibility layer in cache:

function cache($bin = 'cache') {
  // Temporary backwards compatibiltiy layer, allow old style prefixed cache
  // bin names to be passed as arguments.
  $bin = str_replace('cache_', '', $bin);
  // ...

will eventually be removed, we should go through core and remove the last instances of 'cache_FOO' and replace them with 'FOO'.

One place where these type of strings are still present is in hook_flush_caches() and its implementations.

I suggest that we switch from the old style naming to the new style cache bin naming.