Revert "[llvm] Add a way to speed up the speed in which BumpPtrAllocator increases slab sizes"

This reverts commit b848b510 as the unit tests
fail on the sanitizer bots:
/b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/unittests/Support/AllocatorTest.cpp:145: Failure
      Expected: SlabSize
      Which is: 4096
To be equal to: Alloc.getTotalMemory()
      Which is: 4097