Removed legacy MinGW32 AppVeyor CI job.

Boost.Regex develop no longer builds on legacy MinGW32 due to missing
definitions of std::mutex etc.

Author: Lastique

appveyor.yml

@@ -43,10 +43,6 @@ environment:
       CXXSTD: 11,14,1z
       ADDPATH: C:\cygwin64\bin;
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-    - TOOLSET: gcc
-      CXXSTD: 11,14,1z
-      ADDPATH: C:\mingw\bin;
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
     - TOOLSET: gcc
       CXXSTD: 11,14,1z
       ADDPATH: C:\mingw-w64\x86_64-6.3.0-posix-seh-rt_v5-rev1\mingw64\bin;