chore(editorconfig): remove "charset" (neovim#18541)

Hardcoding a charset causes trouble when porting Vim patches.
I previously tried to unset "charset" for certain file extensions, but
vim-patch.sh can generate more files, and automatically detecting file
encoding is more correct anyway.

.editorconfig

@@ -6,14 +6,10 @@ indent_size = 2
 tab_width = 8
 end_of_line = lf
 insert_final_newline = true
-charset = utf-8
 
 [*.{c,h,in,lua}]
 max_line_length = 100
 
-[*.{vim,po}]
-charset = unset
-
 [{Makefile,**/Makefile,runtime/doc/*.txt}]
 indent_style = tab
 indent_size = 8