Do not construct std::function with Allocator argument

Bug: http://b/116702964

This breaks when built with C++17, where these constructors are removed.
Looks like libc++ ignored the allocator arguments anyway - so this
should be NFC even with C++17.

Test: memunreachable_test on the host and Sailfish with C++14, C++17
Change-Id: I638326a900781f64d0562b5ec38e990659967039
diff --git a/libmemunreachable/LeakFolding.cpp b/libmemunreachable/LeakFolding.cpp
index 69f320c..074dc48 100644
--- a/libmemunreachable/LeakFolding.cpp
+++ b/libmemunreachable/LeakFolding.cpp
@@ -57,7 +57,7 @@
 }
 
 void LeakFolding::AccumulateLeaks(SCCInfo* dominator) {
-  std::function<void(SCCInfo*)> walk(std::allocator_arg, allocator_, [&](SCCInfo* scc) {
+  std::function<void(SCCInfo*)> walk([&](SCCInfo* scc) {
     if (scc->accumulator != dominator) {
       scc->accumulator = dominator;
       dominator->cuumulative_size += scc->size;