[nfc][libomptarget] Remove SHARED annotation from local variables

Summary:
[nfc][libomptarget] Remove SHARED annotation from local variables

A few local variables in reduction.cu were marked SHARED. This patch leaves
all per-kernel global state localised in omp_data.cu.

Reviewers: ABataev, jdoerfert, grokos

Reviewed By: jdoerfert

Subscribers: openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D73239