chore(.editorconfig): remove redundant configuration

[dev-itsheng]

1. No any files can match [test/cases/parsing/bom/bomfile.{css,js}] rule, so I remove it.
2. About [*.md], the first rule insert_final_newline = true is repeated as above, the second trim_trailing_whitespace = false, I didn't find the point of applying it, so I removed the entire rule.