MDEV-23024 Remove Cassandra Storage Engine

Cassandra has long been non-functional, doesn't compile, etc.,
and development on it has halted. Since MariaDB Server 10.5,
the build is disabled by default.

It makes sense to remove it as it's just taking up space.

storage/mroonga/appveyor.yml

@@ -31,7 +31,6 @@ build_script:
       -DCMAKE_BUILD_TYPE=Debug
       -DPLUGIN_ARCHIVE=NO
       -DPLUGIN_BLACKHOLE=NO
-      -DPLUGIN_CASSANDRA=NO
       -DPLUGIN_CONNECT=NO
       -DPLUGIN_CSV=NO
       -DPLUGIN_EXAMPLE=NO

storage/mroonga/tools/travis/before_script.sh

@@ -24,7 +24,6 @@ if [ "${MROONGA_BUNDLED}" = "yes" ]; then
   cmake_args=("${cmake_args[@]}" -DWITH_EMBEDDED_SERVER=TRUE)
   cmake_args=("${cmake_args[@]}" -DWITHOUT_ARCHIVE=TRUE)
   cmake_args=("${cmake_args[@]}" -DWITHOUT_BLACKHOLE=TRUE)
-  cmake_args=("${cmake_args[@]}" -DWITHOUT_CASSANDRA=TRUE)
   cmake_args=("${cmake_args[@]}" -DWITHOUT_CONNECT=TRUE)
   cmake_args=("${cmake_args[@]}" -DWITHOUT_CSV=TRUE)
   cmake_args=("${cmake_args[@]}" -DWITHOUT_EXAMPLE=TRUE)