Remove native implementation of the v6 (nv14) actors.

[lemmih]

Summary of changes
Changes introduced in this pull request:

• Removed the DefaultRuntime.
• Removed the native v6 actor implementation.

Reference issue to close (if applicable)

Closes #1552

Other information and links

[codecov]

Codecov Report

Merging #1559 (3406153) into main (8cd7c01) will increase coverage by 13.68%.
The diff coverage is 19.56%.

@@             Coverage Diff             @@
##             main    #1559       +/-   ##
===========================================
+ Coverage   25.61%   39.30%   +13.68%     
===========================================
  Files         289      236       -53     
  Lines       24261    15646     -8615     
===========================================
- Hits         6215     6150       -65     
+ Misses      18046     9496     -8550     

Impacted Files	Coverage Δ
blockchain/state_manager/src/vm_circ_supply.rs	0.00% <0.00%> (ø)
ipld/blockstore/src/lib.rs	42.22% <ø> (+13.33%)	⬆️
utils/statediff/src/lib.rs	0.00% <0.00%> (ø)
vm/interpreter/src/default_runtime.rs	0.00% <ø> (ø)
vm/interpreter/src/downcast.rs	0.00% <0.00%> (ø)
vm/interpreter/src/gas_tracker/mod.rs	89.47% <ø> (ø)
vm/interpreter/src/lib.rs	0.00% <ø> (ø)
vm/actor_interface/src/lib.rs	11.53% <33.33%> (-0.47%)	⬇️
vm/actor_interface/src/adt/map.rs	34.21% <36.84%> (-7.17%)	⬇️
vm/state_tree/src/lib.rs	32.71% <100.00%> (ø)
... and 8 more

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[connormullett]

lgtm