Removed legacy MinGW from AppVeyor CI.

Looks like gcc 5 that is installed in MinGW on AppVeyor CI is not compiled
with C99 standard library enabled, which results in C++11 std::to_string
functions not being available. These functions are required by
Boost.Multiprecision.

Since legacy MinGW is quite outdated and increasingly poorly supported
across Boost libraries, remove it from CI rather than adding yet another
special case for disabling Boost.Multiprecision tests.

appveyor.yml

@@ -42,10 +42,6 @@ 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;