LangRef: Remove stale docs on LLVM types in module structure

The distinction between "identified" and "literal" struct types is fully
documented in a later section.

Patch by Philip Reames!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202927 91177308-0d34-0410-b5e6-96231b3b80d8