[libc] Remove leftover 'gpu/' source directory (llvm#122368)

Summary:
This isn't used anymore, I moved the GPU extensions into `offload/`.

Author: jhuber6

libc/src/CMakeLists.txt

@@ -26,10 +26,6 @@ if(${LIBC_TARGET_OS} STREQUAL "linux")
   add_subdirectory(unistd)
 endif()
 
-if(${LIBC_TARGET_OS} STREQUAL "gpu")
-  add_subdirectory(gpu)
-endif()
-
 if(NOT LLVM_LIBC_FULL_BUILD)
   return()
 endif()