Remove native smart-contract backend.

[lemmih]

Summary of changes
Changes introduced in this pull request:

• Remove the unused code for the native (non-fvm) execution backend.

Reference issue to close (if applicable)

Other information and links

[codecov]

Codecov Report

Merging #1542 (d76b1e4) into main (d846152) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1542      +/-   ##
==========================================
+ Coverage   25.61%   25.63%   +0.01%     
==========================================
  Files         289      288       -1     
  Lines       24261    24246      -15     
==========================================
+ Hits         6215     6216       +1     
+ Misses      18046    18030      -16     

Impacted Files	Coverage Δ
vm/interpreter/src/vm.rs	0.00% <0.00%> (ø)
blockchain/message_pool/src/msg_chain.rs	80.64% <0.00%> (+0.40%)	⬆️

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[LesnyRumcajs]

RIP, forever in our hearts (and git history) 🕯

[connormullett]

lgtm!