[Headers] Remove musl-related comment about NULL

This removes a comment added in D159312, which warned people to not
re-add a whitespace in the `((void*)0))` expression. After discussions
happened in D159312, it doesn't seem like a permanent solution.

While I'd like to keep the whitespace removed for now, given that at
least it can be a band-aid to some users who use musl and clang's
`stddef.h` at the same time, it seems the usage of them together is not
something that's officially supported, and I should not be implying this
should be the permanent solution by saying so in the comments.

Reviewed By: aaron.ballman, ributzka

Differential Revision: https://reviews.llvm.org/D159383