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
1 file changed