nsis: Remove NSIS Installer Code

nsis: Remove NSIS Installer Code

Move the NSIS code to the vim/vim-win32-installer repository.
When we update the NSIS code, we sometimes need to update the
vim/vim-win32-installer scripts also. Managing them in the same repository
should be better for maintenance.

This PR should be merged after https://github.com/vim/vim-win32-installer/pull/457

closes: #20521

Signed-off-by: K.Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>