vim:keys: Add <leader>dd to clear line

I sometimes have the situation that I want to create empty lines but
they contain either whitespace from keeping the indentation and/or the
comment string (`o` on a comment line). My current workflow is to either
delete it and recreate it properly (e.g. from the 'other side' i.e. a
non-comment line) or clear it with `cc`. Both ways are a bit awkward.

Add a mapping that clears a line but stays in normal mode.

Also remove the `<leader>d` mapping that deletes and puts the content
into the system clipboard as I rarely use this one.

.config/vim/vimrc.d/40-keys.vim

@@ -28,7 +28,6 @@ endif
 if (has('clipboard'))
 	map <leader>y "+y
 	map <leader>Y "+Y
-	map <leader>d "+d
 	map <leader>p "+p
 	map <leader>P "+P
 endif
@@ -149,6 +148,9 @@ endif
 " Y should behave like D & C does
 nnoremap Y y$
 
+" Clear line (`cc` but stay in normal mode)
+nmap <leader>dd ^D
+
 " Move lines up and down while correcting the indentation
 " https://vim.fandom.com/wiki/Moving_lines_up_or_down
 " (Use arrows, as Alt-{j,k} is used by my terminal for scrollback)