Merge bitcoin#805: Remove the extremely outdated TODO file.

1c32519 Remove the extremely outdated TODO file. (Gregory Maxwell)

Pull request description:

  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.

ACKs for top commit:
  real-or-random:
    ACK bitcoin-core/secp256k1@1c32519

Tree-SHA512: 65c730ad2816b28991cdb74df6da4671abe76a74a0d0b306f13612b4bbe9b54f9a623b18fc288e0ec13572d9fdbab6f376ce7aafc9fe601644239629b84fb15c