Remove `dtoa.c/h`

[codereport]

[zhihaoy]

J currently does not do perfect round-trip floating formatting.
The bundled dtoa code is used in the foreign 8 https://www.jsoftware.com/help/dictionary/dx008.htm It may be possible to replace those with some libraries such as dragonbox.
I added perfect round-trip floating formatting to the J fork for my employer using dragonbox https://github.com/zhihaoy/jsource/blob/jruntime-pyd/jsrc/cvr.cpp