Revert "editorconfig: punish tab usage"

This reverts commit 3840c98.
There are multiple reasons:
- the correct way to prevent tabs being mistakenly added is a CI linting step
- it can affect the editing experience of foreign source files
  under mpv's tree (e.g. subprojects)
- at least one editor (BBEdit) misinterprets the values and also turns
  soft tabs into 7 spaces
- it does not consistently apply to GH code view either

.editorconfig

@@ -8,7 +8,6 @@ charset = utf-8
 insert_final_newline = true
 indent_style = space
 indent_size = 4
-tab_width = 7
 max_line_length = 80
 trim_trailing_whitespace = true