fs: Quietly ignore non-dir targets in get_files_in_dir() (BFS only)

The non-BFS implementation already ignored this case without making
noise in stderr, and this function is used by delete_directory(), which
also accepts both regular files and directories. The add-ons management
code has always relied on this behavior.
Revert "Use delete_file() to delete reg files in config_cache::delete_cache_files()"

This reverts commit d06c39b714ef48e5e6dc96bc62a73c994f6e8c45.