[libc++] Remove unused HAVE_LIBCXXABI variable from Android cache

This variable isn't used anymore in libc++'s build and hasn't been
for a few years, so this is likely a remnant of the past.

Author: ldionne

clang/cmake/caches/Android.cmake

@@ -9,7 +9,6 @@ set(CLANG_VENDOR Android CACHE STRING "")
 
 set(CMAKE_BUILD_TYPE RELEASE CACHE STRING "")
 
-set(HAVE_LIBCXXABI ON CACHE BOOL "")
 set(LLVM_BUILD_TOOLS OFF CACHE BOOL "")
 set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
 set(LLVM_ENABLE_THREADS OFF CACHE BOOL "")