remove duplicate initialisations of global variables
return to vundle_default_git_proto to protect user settings

It is not possible to initialize vundle#default_git_proto in
autoload/vundle.vim together with the other global variables as this might
override the users setting from the vimrc file. (at least such an
initialization as to be protected with an extra test).

Therefor this change will be postponed until the name of this variable is
changed (see discussion in #450).