Remove glslang::pool_allocator::setAllocator

TPoolAllocator is not copy assignable, so this setter could never have
been used. After a recent change (878a24ee2), new versions of Clang
reject this code outright.

(cherry picked from commit 24b3e8384e93f3e73b6aa14ea00a30574112f9ba)
Change-Id: If2390189378c4184b8e71b9a2ceb6d2b32047f9e
diff --git a/glslang/Include/PoolAlloc.h b/glslang/Include/PoolAlloc.h
index 0e237a6..b8eccb8 100644
--- a/glslang/Include/PoolAlloc.h
+++ b/glslang/Include/PoolAlloc.h
@@ -304,7 +304,6 @@
     size_type max_size() const { return static_cast<size_type>(-1) / sizeof(T); }
     size_type max_size(int size) const { return static_cast<size_type>(-1) / size; }
 
-    void setAllocator(TPoolAllocator* a) { allocator = *a; }
     TPoolAllocator& getAllocator() const { return allocator; }
 
 protected: