Remove dead code

This is a rebase of #236.

HAVE_STACK_LIMIT definition was commented out in the original Mercurial repo:
https://code.google.com/p/vim/source/detail?r=7e70fc748752

...and then completely removed:
https://code.google.com/p/vim/source/detail?r=6768ebd0bc04

Custom undo memory allocator was removed:
https://code.google.com/p/vim/source/detail?r=290ee42cae85