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

src/dictionary.cc

@@ -15,7 +15,6 @@
 #include <algorithm>
 #include <iterator>
 #include <unordered_map>
-#include <cctype>
 
 namespace fasttext {
 

src/fasttext.cc

@@ -9,7 +9,6 @@
 
 #include "fasttext.h"
 
-#include <fenv.h>
 #include <math.h>
 
 #include <iostream>

src/utils.cc

@@ -9,7 +9,6 @@
 
 #include "utils.h"
 
-#include <cmath>
 #include <ios>
 
 namespace fasttext {