Remove TODO.md

The file predates open-sourcing and we managed to implement most
of it by now. The remaining features have tracking issues.

Fixes #1155

TODO.md

@@ -1,19 +0,0 @@
-
-- [ ] completion isIncomplete support
-- [ ] respect view fullscreen flag
-- [ ] Implement marks (superset of Selection/Range)
-
-- [ ] = for auto indent line/selection
-- [ ] lsp: signature help
-
-2
-- [ ] store some state between restarts: file positions, prompt history
-- [ ] highlight matched characters in picker
-
-3
-- [ ] diff mode with highlighting?
-- [ ] snippet support (tab to jump between marks)
-- [ ] gamelisp/wasm scripting
-
-X
-- [ ] rendering via skulpin/skia or raw wgpu