[OpenMP][FIX] Do not adjust the level after the environment was popped

[jdoerfert]

Summary

Exiting a data environment will reset all values, it is wrong to adjust
them afterwards.

[tianshilei1992]

LGTM. Or to make it symmetrical, which is pointless though.

void __kmpc_end_serialized_parallel(IdentTy *, uint32_t TId) {
  --icv::Level;
  state::exitDataEnvironment();
}

openmp/libomptarget/DeviceRTL/src/Parallelism.cpp

 
 void __kmpc_serialized_parallel(IdentTy *, uint32_t TId) {
   state::enterDataEnvironment();
   ++icv::Level;
 }
 
 void __kmpc_end_serialized_parallel(IdentTy *, uint32_t TId) {
   state::exitDataEnvironment();
-  --icv::Level;
 }
 
 uint16_t __kmpc_parallel_level(IdentTy *, uint32_t) { return omp_get_level(); }
 
 int32_t __kmpc_global_thread_num(IdentTy *) { return omp_get_thread_num(); }
 
 void __kmpc_push_num_threads(IdentTy *, int32_t, int32_t NumThreads) {
   icv::NThreads = NumThreads;