[.editorconfig] Remove trim_trailing_whitespace

Forcing this to be `false` is unhelpful for editors
that allow incremental removal of trailing whitespace
as you edit, such as Xcode. For now, avoid enforcing
the setting, leaving it up to individual editor
configurations. This restores the behavior prior to
the addition of the `.editorconfig`.

Author: hamishknight

.editorconfig

@@ -5,5 +5,4 @@ root = true
 [*]
 indent_style = space
 indent_size = 2
-trim_trailing_whitespace = false
 insert_final_newline = true