Undo unintended change to flang-rt cmake file.

Author: DanielCChen

flang-rt/cmake/modules/GetToolchainDirs.cmake

@@ -118,8 +118,6 @@ function (get_toolchain_arch_dirname outvar)
     set(target "amdgcn-amd-amdhsa")
   elseif("${arch}" MATCHES "^nvptx")
     set(target "nvptx64-nvidia-cuda")
-  elseif(UNIX AND CMAKE_SYSTEM_NAME MATCHES "AIX")
-    string(TOLOWER "${CMAKE_SYSTEM_NAME}" target)
   else()
     set(target "${arch}${triple_suffix}")
   endif()