redundant static binaries for Boost Shared builds on Windows

Hello, everyone,

For Boost Shared builds on Windows (mean `link=shared` build key) static binaries are created in addition to shared. E.g. Boost installation folder for builds using mingw-w64 with Shared layout contain files:
```
libboost_chrono-mgw63-mt-1_64.a <==
libboost_chrono-mgw63-mt-1_64.dll
libboost_chrono-mgw63-mt-1_64.dll.a
libboost_system-mgw63-mt-1_64.a <==
libboost_system-mgw63-mt-1_64.dll
libboost_system-mgw63-mt-1_64.dll.a
libboost_timer-mgw63-mt-1_64.a <==
libboost_timer-mgw63-mt-1_64.dll
libboost_timer-mgw63-mt-1_64.dll.a
libboost_unit_test_framework-mgw63-mt-1_64.a <==
libboost_unit_test_framework-mgw63-mt-1_64.dll
libboost_unit_test_framework-mgw63-mt-1_64.dll.a
```

and using ICC or MSVC:
```
boost_chrono-vc140-mt-1_64.dll
boost_chrono-vc140-mt-1_64.lib
boost_system-vc140-mt-1_64.dll
boost_system-vc140-mt-1_64.lib
boost_timer-vc140-mt-1_64.dll
boost_timer-vc140-mt-1_64.lib
boost_unit_test_framework-vc140-mt-1_64.dll
boost_unit_test_framework-vc140-mt-1_64.lib
libboost_chrono-vc140-mt-1_64.lib <==
libboost_system-vc140-mt-1_64.lib <==
libboost_timer-vc140-mt-1_64.lib <==
libboost_unit_test_framework-vc140-mt-1_64.lib <==
```
Reproduced for Boost.Chrono, Boost.System, Boost.Timer, Boost.Unit_test_framework modules only.

~~Could this static builds be removed from Boost Shared builds?~~
Could rules for building static binaries for Shared builds be removed from Boost build system?

Environment:
- Windows 10 x64,
- mingw-w64 6.3.0
- Intel Parallel Studio XE 2017 Update 4,
- Visual Studio 2015 Update 3,
- BOOST-1.64.0.

Best,

Alexander