Remove the extremely outdated TODO file.

Summary:
This had two things in it-- tests for the scalar/field code and
constant time signing and keygen.

The signing and keygen have been thoroughly constant time for years
and there are now powerful tests to verify it... no further work
on constant-time is needed at least on ordinary platforms (other
sidechannels-- sure).

The scalar and field code have extensive tests. They could use
better static test vectors but they're well tested.

TODOs for the project are currently better documented on github
right now. This file could return in the future with current
info, if needed.

This is a backport of secp256k1 PR805

Test Plan:

\o/

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7606