.editorconfig: remove wrong key tab_size

according to https://editorconfig.org/#supported-properties it should be `tab_width`, but it anyhow defaults to the value of `indent_size`

Author: mguetschow

.editorconfig

@@ -11,7 +11,6 @@ insert_final_newline = true
 [{Makefile*,*.mk}]
 indent_style = tab
 indent_size = 2
-tab_size = 2
 trim_trailing_whitespace = true
 insert_final_newline = true