Remove redundancy in Global/Vim.gitignore

Anything matched by `*.un~` will also be matched by the last rule, `*~`.

Global/Vim.gitignore

 # swap
 [._]*.s[a-w][a-z]
 [._]s[a-w][a-z]
-# persistent undo 
-*.un~
 # session
 Session.vim
 # temporary