clang/win: Remove #pragma hdrstop workaround.

clang-cl supports #pragma hdrstop as of r341963.

Bug: 505314,504657,750327,826743
Change-Id: Ic90b07114bbb886b7d8345a260863f45eef352b7
Reviewed-on: https://chromium-review.googlesource.com/c/1317607
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605445}

third_party/unrar/src/global.cpp

 #define INCLUDEGLOBAL
 
-#if !defined(CHROMIUM_UNRAR)
 #if defined(__BORLANDC__) || defined(_MSC_VER)
 #pragma hdrstop
 #endif
-#endif  // defined(CHROMIUM_UNRAR)
 
 #include "rar.hpp"