bugtool: simplify removeIfEmpty with more efficient os.ReadDir

[Juneezee]

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We can simplify the following code

dir, err := os.Open(dirname)
if err != nil {
	return err
}
defer dir.Close()

dirs, err := dir.Readdir(-1)

with just os.ReadDir(dirname). os.ReadDir is recommended over Readdir(), as stated in the official doc ¹

bugtool: simplify `removeIfEmpty` with more effiicient `os.ReadDir`

Footnotes

1. https://pkg.go.dev/os#File.Readdir ↩