Fix vim textwidth in nested project commit message

Unsetting the max_line_length rule here for all nested projects causes vim, via the editorconfig-vim plugin, to set its textwidth option to zero* when editing commit messages in those nested projects, meaning that it no longer hard line wraps git commit messages.

Normally, vim's textwidth option is set to 72 for git commit messages.

Since nowhere else in this file sets the max_line_length rule, there's little point in unsetting it. Removing this line should therefore have no other effect than working around this issue with vim.

*: Actually, it sets textwidth to whatever max_line_length is, which for some reason resolves to zero when unset, rather than using the editor default.