Use blackhole register when removing lines

During session save, when removing lines, we don't want to store the content
in the unnamed register, takes more time and changes clipboard content. Just
use the blackhole register.

References #167.

autoload/startify.vim

@@ -268,7 +268,7 @@ function! startify#session_write(spath)
     " remove lines from the session file
     if exists('g:startify_session_remove_lines')
       for pattern in g:startify_session_remove_lines
-        execute 'silent global/'. pattern .'/delete'
+        execute 'silent global/'. pattern .'/delete _'
       endfor
     endif