transform.RemoveColumn returns nil error when column doesn't exist

## Description
`transform.RemoveColumn("nonexistent")` succeeds silently when the column doesn't exist in the query.

## Reproduction
```go
rule := transform.RemoveColumn("nonexistent")
result, err := transform.Apply("SELECT id, name FROM users", rule)
// err is nil, result unchanged
```

## Expected
Return an error indicating the column was not found.