Don't attempt Windows cross-compilation

Well that was disappointing. Ubuntu doesn't seem to have packages
for MinGW-compiled libs, so I can't satisfy the build dependencies.

I might try just doing native Windows compilation, since I'd have
MSYS2 at my disposal.

.travis.yml

@@ -36,7 +36,6 @@ addons:
         - gcc-9
         - g++-9
         - clang-9
-        - mingw-w64
         - libsdl2-dev
         - libfreetype6-dev
         - libfltk1.3-dev
@@ -130,18 +129,6 @@ script:
     - make -j ${JOBS} FIX_BUGS=1 ${MAKE_BUILD_TYPE} WARNINGS=1 WARNINGS_ALL=1
     - cd ${CMAKE_BUILD_DIR}
 
-    # Windows build
-    - |
-      if [ "$TRAVIS_OS_NAME" = "linux" ]; then
-        if [ "$CXX" = "g++" ]; then
-          # Compile 32-bit build
-          make -j ${JOBS} FIX_BUGS=1 ${MAKE_BUILD_TYPE} WARNINGS=1 WARNINGS_ALL=1 WINDOWS=1 STATIC=1 CC=i686-w64-mingw32-gcc CXX=i686-w64-mingw32-g++ WINDRES=i686-w64-mingw32-windres
-
-          # Compile 64-bit build
-          make -j ${JOBS} FIX_BUGS=1 ${MAKE_BUILD_TYPE} WARNINGS=1 WARNINGS_ALL=1 WINDOWS=1 STATIC=1 CC=x86_64-w64-mingw32-gcc CXX=x86_64-w64-mingw32-g++ WINDRES=x86_64-w64-mingw32-windres
-        fi
-      fi
-
 after_success:
     # Send success notification to Discord through DISCORD_WEBHOOK_URL
     - travis_retry wget ${DISCORD_SEND_SCRIPT_URL} -O ${DISCORD_SEND_SCRIPT_FILENAME}