patchgit-gui: simplify PATH de-duplication

> git-gui since 8fe7861c51 ("git-gui: assure PATH has only absolute
> elements.", 2025-04-11) uses a list to maintain order and a dict to
> detect duplicated elements without quadratic complexity.  But, Tcl's
> dict explicitly maintains keys in the order first added, thus the list
> is not needed.  Simplify the code.
> 
> Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
> ---
>  git-gui.sh | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/git-gui.sh b/git-gui.sh
> index b64d065..4528b22 100755
> --- a/git-gui.sh
> +++ b/git-gui.sh
> @@ -103,21 +103,16 @@ if {[is_Windows]} {
>  	set _path_sep {:}
>  }
>  
> -set _search_path {}
>  set _path_seen [dict create]
>  foreach p [split $env(PATH) $_path_sep] {
>  	# Keep only absolute paths, getting rid of ., empty, etc.
>  	if {[file pathtype $p] ne {absolute}} {
>  		continue
>  	}
> -	# Keep only the first occurence of any duplicates.
>  	set norm_p [file normalize $p]
> -	if {[dict exists $_path_seen $norm_p]} {
> -		continue
> -	}
>  	dict set _path_seen $norm_p 1
> -	lappend _search_path $norm_p
>  }
> +set _search_path [dict keys $_path_seen]
>  unset _path_seen
>  
>  set env(PATH) [join $_search_path $_path_sep]