ci: remove libass enablement

This is no longer a configurable option.

TOOLS/appveyor-build.sh

@@ -16,7 +16,6 @@ export PYTHON=/usr/bin/python3
     --enable-jpeg \
     --enable-lcms2 \
     --enable-libarchive \
-    --enable-libass \
     --enable-lua \
     --enable-rubberband \
     --enable-shaderc \

ci/build-mingw64.sh

@@ -22,7 +22,6 @@ python3 ./waf configure \
     --enable-lua \
     --enable-javascript \
     --enable-libarchive \
-    --enable-libass \
     --enable-libbluray \
     --enable-dvdnav \
     --enable-uchardet \