Revert "[CAS] Add OnDiskDataAllocator (#161264)"

This reverts commit 08e9540.

Doesn't build on some bots,
see comments on #161264

Author: nico

llvm/include/llvm/CAS/OnDiskDataAllocator.h

@@ -7,7 +7,6 @@ add_llvm_component_library(LLVMCAS
   MappedFileRegionArena.cpp
   ObjectStore.cpp
   OnDiskCommon.cpp
-  OnDiskDataAllocator.cpp
   OnDiskTrieRawHashMap.cpp
 
   ADDITIONAL_HEADER_DIRS

llvm/lib/CAS/CMakeLists.txt

@@ -8,7 +8,6 @@ add_llvm_unittest(CASTests
   ActionCacheTest.cpp
   CASTestConfig.cpp
   ObjectStoreTest.cpp
-  OnDiskDataAllocatorTest.cpp
   OnDiskTrieRawHashMapTest.cpp
   ProgramTest.cpp
   )