Appveyor: remove redundant CFLAGS

Also changed -Wno-error=implicit-fallthrough to
-Wno-implicit-fallthrough. For some reason Appveyor's
version of GCC is triggering these warnings despite the
comments, so just shut it up.

appveyor.yml

@@ -6,7 +6,7 @@ environment:
  WINDRES: windres
  # c:\mingw-w64 i686 has gcc 6.3.0, so use c:\msys64 7.3.0 instead
  MINGW_SDK: c:\msys64\mingw32
- CFLAGS: -Wall -W -Werror -Wno-error=implicit-fallthrough -Wimplicit-fallthrough=3 -Wno-tautological-compare -Wno-error=suggest-attribute=noreturn
+ CFLAGS: -Wno-implicit-fallthrough
  NASM_ZIP: nasm-2.12.01
  NASM_URL: http://www.nasm.us/pub/nasm/releasebuilds/2.12.01/win64/nasm-2.12.01-win64.zip
  UPX_ZIP: upx391w