Removed last boost/thread.hpp include

Locale doesn't rely on boost, so it must have been some other issue in Boost 1.60
that was incidentally fixed by including that file. If anyone ever happens to build
with 1.60 again we can revisit this.

For the record, there are no issues with VS 2017 and Boost 1.66.

(cherry-picked from commit 7cf9341)

src/gettext_boost.cpp

@@ -23,8 +23,6 @@
 #include <locale>
 #include <mutex>
 #include <boost/locale.hpp>
-// including boost/thread fixes linking of boost locale for msvc on boost 1.60
-#include <boost/thread.hpp>
 #include <set>
 
 #if defined(__GNUC__)