Rip out dependabot - its worse than useless - its annoying

[TheBlueMatt]

Dependabot has a ton of issues with its rust integration that makes
it wholly useless, and very annoying:

• It has no concept of MSRV, opening PRs that are not going to pass
  CI.
• It has no concept of patch-level - if we depend on tokio 1.X,
  that means any version of tokio > 1.X, but dependabot insists on
  opening a PR to "update us" to tokio 1.X + 1, even though it
  doesn't impact what version of our users use (and often violates
  MSRV).
• It has no concept of dependencies that rely on each other,
  causing it to open a PR to update us to bitcoin_hashes X + 1,
  even though we're still depending on rust-bitcoin Y which
  depends on bitcoin_hashes X, causing build failure.
• It hogs CI resources, getting CI run twice, once for the branch
  once for the PR.
• It creates branches directly on the rust-lightning repo, making
  it look like the work is somehow connected to the
  lightningdevkit project, even though it isn't, and spamming the
  local clones of project contributors.

At the end of the day, dependabot has never meaningfully
contributed to notifying us of an important dependency, and,
really, we don't have enough dependencies for it to matter.

[codecov-commenter]

Codecov Report

Merging #1615 (0a8bfde) into main (f3d5b94) will decrease coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 0a8bfde differs from pull request most recent head a911ca8. Consider uploading reports for the commit a911ca8 to get more accurate results

@@            Coverage Diff             @@
##             main    #1615      +/-   ##
==========================================
- Coverage   91.07%   91.03%   -0.05%     
==========================================
  Files          80       80              
  Lines       44128    44128              
  Branches    44128    44128              
==========================================
- Hits        40190    40171      -19     
- Misses       3938     3957      +19     

Impacted Files	Coverage Δ
lightning/src/ln/payment_tests.rs	98.88% <0.00%> (-0.38%)	⬇️
lightning-net-tokio/src/lib.rs	76.85% <0.00%> (-0.31%)	⬇️
lightning/src/ln/functional_tests.rs	96.87% <0.00%> (-0.24%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f3d5b94...a911ca8. Read the comment docs.

[TheBlueMatt]

Lol, fixed git commit to say "dependabot", not "dependable".

[tnull]

RIP