fix: Remove invalid file 'index.'

[matwilko]

This file was introduced in 1a23372, presumably mistakenly since it's empty.

It doesn't appear to be affecting builds or anything, but it does prevent the repo from being checked out on Windows systems, where filenames are not permitted to end with a ., e.g.:

Resolving deltas: 100% (76774/76774), done.
error: invalid path 'packages/plugin-minify/lib/remove-var-undefined/index.'
fatal: unable to checkout working tree
warning: Clone succeeded, but checkout failed.

[coderaiser]

Thanks!