Drop fuzztarget feature entirely

[TheBlueMatt]

Some time ago we started transitioning to cfg(fuzzing) instead of
exposing a full feature. Here we complete the transition.

[codecov-commenter]

Codecov Report

Merging #1316 (d7bb532) into main (8e7f241) will increase coverage by 0.14%.
The diff coverage is 100.00%.

❗ Current head d7bb532 differs from pull request most recent head b0cb558. Consider uploading reports for the commit b0cb558 to get more accurate results

@@            Coverage Diff             @@
##             main    #1316      +/-   ##
==========================================
+ Coverage   90.52%   90.66%   +0.14%     
==========================================
  Files          72       72              
  Lines       39560    40288     +728     
==========================================
+ Hits        35811    36527     +716     
- Misses       3749     3761      +12     

Impacted Files	Coverage Δ
lightning/src/chain/chainmonitor.rs	97.64% <ø> (ø)
lightning/src/lib.rs	100.00% <ø> (ø)
lightning/src/ln/chan_utils.rs	95.51% <ø> (ø)
lightning/src/ln/channel.rs	89.33% <ø> (ø)
lightning/src/ln/mod.rs	95.00% <ø> (ø)
lightning/src/ln/msgs.rs	85.88% <ø> (ø)
lightning/src/routing/router.rs	91.92% <ø> (ø)
lightning/src/util/chacha20.rs	96.37% <ø> (ø)
lightning/src/util/chacha20poly1305rfc.rs	97.95% <ø> (ø)
lightning/src/util/fuzz_wrappers.rs	100.00% <ø> (ø)
... and 4 more

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