Remove dead code. NFC.

This interface was added 2 years ago but users never developed.

llvm-svn: 223368
diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index 7eb0752..539b6c3 100644
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -330,7 +330,7 @@
 getOpenFileImpl(int FD, const Twine &Filename, uint64_t FileSize,
                 uint64_t MapSize, int64_t Offset, bool RequiresNullTerminator,
                 bool IsVolatileSize) {
-  static int PageSize = sys::process::get_self()->page_size();
+  static int PageSize = sys::Process::getPageSize();
 
   // Default is to map the full file.
   if (MapSize == uint64_t(-1)) {