[libc++] Remove redundant conditionals for Apple platforms

Summary:
In a bunch of places, we used to check whether LIBCXX_BUILDING_LIBCXXABI
is defined OR we're building for an Apple platform. This used to
be necessary in a time when Apple's build script did NOT define
LIBCXX_BUILDING_LIBCXXABI. However this is not relevant anymore
since Apple's build does define LIBCXX_BUILDING_LIBCXXABI.

Reviewers: EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60842

llvm-svn: 358988
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: d524fe5b3a0975a1c037669ade35c182681c14f9
diff --git a/src/exception.cpp b/src/exception.cpp
index ee9f384..fce6db7 100644
--- a/src/exception.cpp
+++ b/src/exception.cpp
@@ -10,8 +10,7 @@
 #include "new"
 #include "typeinfo"
 
-#if defined(LIBCXXRT) || defined(LIBCXX_BUILDING_LIBCXXABI) || \
-  (defined(__APPLE__) && !defined(_LIBCPP_BUILDING_HAS_NO_ABI_LIBRARY))
+#if defined(LIBCXXRT) || defined(LIBCXX_BUILDING_LIBCXXABI)
   #include <cxxabi.h>
   using namespace __cxxabiv1;
   #define HAVE_DEPENDENT_EH_ABI 1