Remove now-useless FOLLY_MALLOC_H_

Remove now-useless FOLLY_MALLOC_H_

Summary:
Malloc.h is intentionally included multiple times, once from folly and once from libstdc++. The current ###pragma once## implicitly allows this to happen. FBString.h has an undef for Malloc.h's include guard, which originally accomplished this goal.

The undefing code is presently moot, since its functionality has been replaced by ###pragma once##. Remove it.

I noticed this when ott was copying FBString over to libstdc++ in D3757853.
The diff that switched the include guards to pragmas was a codemod, and was not specific to fbstring. D3054492

Reviewed By: ot

Differential Revision: D3758119

fbshipit-source-id: e796d039a031d5f842ed39bf55a6b1aeb2686bc4