Removed MinGW-w64 gcc 6.3 job from AppVeyor.

The compiler does not provide synchronization lib, which is needed by
Boost.Atomic.

Author: Lastique

appveyor.yml

@@ -40,9 +40,6 @@ environment:
     - ARGS: toolset=gcc address-model=32 cxxstd=11,14
       PATH: C:\mingw-w64\i686-5.3.0-posix-dwarf-rt_v4-rev0\mingw32\bin;%PATH%
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-    - ARGS: toolset=gcc address-model=64 cxxstd=11,14,1z
-      PATH: C:\mingw-w64\x86_64-6.3.0-posix-seh-rt_v5-rev1\mingw64\bin;%PATH%
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
     - ARGS: toolset=gcc address-model=64 cxxstd=11,14,1z
       PATH: C:\mingw-w64\x86_64-7.3.0-posix-seh-rt_v5-rev0\mingw64\bin;%PATH%
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015