Remove MinGW (non w64) from Appveyor

Author: pdimov

appveyor.yml

@@ -42,11 +42,6 @@ environment:
       TOOLSET: gcc
       VARIANT: release
 
-    - ADDPATH: C:\mingw\bin;
-      TOOLSET: gcc
-      CXXSTD: 11
-      VARIANT: debug
-
     - ADDPATH: C:\cygwin64\bin;
       TOOLSET: gcc
       VARIANT: debug
@@ -57,7 +52,7 @@ environment:
 
     - ADDPATH: C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin;
       TOOLSET: gcc
-      VARIANT: debug,release
+      VARIANT: release
 
 install:
   - set GIT_FETCH_JOBS=8