remove unused includes in fbcode (skipping #if, new default mode)

Summary:
This solves most include warnings as seen in Phabricator (no header files, no "packing" system headers, new default mode where more user headers are removed).

We cowardly skip files containing #if for now.

Generated by
```
rm -f /tmp/ffmr-diff/* &&
cd fbcode &&
(foundation/scripts/ls-cpp-dirs | grep -v '^\(\.\.\|external/\|.*/external\|folly/|watchman/\)' |
xargs ffmr -o /tmp/ffmr-diff codegraph/scripts/ffmr/analyze_includes_no_headers_no_packing_skipping_if.sh) &&
(cat /tmp/ffmr-diff/*.diff | patch -p2) &&
hg commit -m foo &&
cd .. &&
arc amend --yes --revision D4414676 && arc diff --nolint --nounit --excuse refactoring --prepare --big-diff -m 'something'
```

folly and watchman are in separate diffs.

Reviewed By: meyering

Differential Revision: D4414676

fbshipit-source-id: 75e2e11f4fac8a5f8071a1bafcc4ddc355fd6f4e

src/model.cc

@@ -13,8 +13,6 @@
 
 #include <algorithm>
 
-#include "utils.h"
-
 namespace fasttext {
 
 Model::Model(std::shared_ptr<Matrix> wi,

src/vector.cc

@@ -14,7 +14,6 @@
 #include <iomanip>
 
 #include "matrix.h"
-#include "utils.h"
 
 namespace fasttext {