Remove a bunch of unused functionalty from agi::Exception

automation/tests/aegisub.cpp

libaegisub/common/character_count.cpp

libaegisub/common/file_mapping.cpp

libaegisub/common/option_visit.cpp

libaegisub/common/option_visit.h

libaegisub/common/path.cpp

libaegisub/include/libaegisub/charset.h

libaegisub/include/libaegisub/charset_conv.h

libaegisub/include/libaegisub/exception.h

libaegisub/include/libaegisub/fs.h

libaegisub/include/libaegisub/io.h

libaegisub/include/libaegisub/keyframe.h

libaegisub/include/libaegisub/lua/utils.h

libaegisub/include/libaegisub/mru.h

libaegisub/include/libaegisub/option.h

libaegisub/include/libaegisub/option_value.h

libaegisub/include/libaegisub/vfr.h

libaegisub/lua/modules/lfs.cpp

libaegisub/lua/script_reader.cpp

src/ass_dialogue.cpp