wmllint: revert part of commit 61648041284627e299ddf26bc601009520c5198e

groggy: The ifdef_stack = [None] assignment made wmllint crash upon nested
#if blocks. The following block of wml should suffice to let it crash. The
inner #endif deletes the data about encountered #ifs.

...
#ifver WESNOTH_VERSION >= 1.11.0
#ifhave ~add-ons/UMC_Music_Book_1/_main.cfg
[binary_path]
	path=data/add-ons/UMC_Music_Book_1
[/binary_path]
#endif
#else
#ifhave ~add-ons/UMC_Music/_main.cfg
[binary_path]
	path=data/add-ons/UMC_Music
[/binary_path]
#endif
#endif
...