fbcode: remove unused includes from .cpp files with no #if and #define

Summary:
This is a first diff to remove the "easiest" unused includes in fbcode.

* For safety, we only touch .cpp files without #if and #define,
* We do not try to remove redundant systems headers (aka. "packing").

The diff was generated as follows:
```
foundation/scripts/ls-cpp-dirs | grep -v '^\(\.\.\|external/\|.*/external\)' | xargs ffmr -o /tmp/ffmr-diff-1 codegraph/scripts/ffmr/analyze_includes_no_headers_no_packing_skipping_ifdefs.sh

cat /tmp/ffmr-diff-1/*.diff | patch -p2
hg commit -m something
arc diff --prepare --nolint --nounit --less-context --excuse refactoring
```

Note: `grep -v` is just an optimization. The actual configuration is in these two files:
diffusion/FBS/browse/master/fbcode/codegraph/analysis/config.py
diffusion/FBS/browse/master/fbcode/codegraph/scripts/ffmr/analyze_includes_no_headers_no_packing_skipping_ifdefs.sh

See the task for more context, and the recent "safety" improvements on the tool.

depends on D4317825 for very few cases where `nolint` had to be manually added.

Reviewed By: igorsugak

Differential Revision: D4312617

fbshipit-source-id: ecc1f0addfd0651fa4770fcc43cd1314661a311a

CounterStatistic.cpp

@@ -12,7 +12,6 @@
 
 #include "treadmill/Util.h"
 
-#include <cmath>
 #include <mutex>
 #include <unordered_map>
 

services/memcached/Treadmill.cpp

@@ -8,8 +8,6 @@
  *
  */
 
-#include <gflags/gflags.h>
-
 #include "treadmill/services/memcached/Connection.h"
 #include "treadmill/services/memcached/Workload.h"
 

services/sleep/sleepserver/SleepClient.cpp

@@ -8,7 +8,6 @@
  *
  */
 
-#include <unistd.h>
 #include <gflags/gflags.h>
 
 #include "treadmill/services/sleep/gen-cpp2/Sleep.h"