runtime: Deprecate transaction::MethodDispatcher

[Yawning]

By deprecate I mean, move into the simple-keyvalue runtime's main.rs,
preserving existing functionality in the worst possible way.

Part of #3572

[codecov]

Codecov Report

Merging #4198 (3fdac0d) into master (5a88c9b) will decrease coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4198      +/-   ##
==========================================
- Coverage   69.28%   68.74%   -0.54%     
==========================================
  Files         411      411              
  Lines       47420    47420              
==========================================
- Hits        32857    32601     -256     
- Misses      10525    10795     +270     
+ Partials     4038     4024      -14     

Impacted Files	Coverage Δ
...nsensus/tendermint/apps/keymanager/transactions.go	0.00% <0.00%> (-52.95%)	⬇️
go/ias/http/http.go	20.33% <0.00%> (-44.07%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/keymanager/api/policy_sgx.go	33.33% <0.00%> (-13.34%)	⬇️
go/runtime/host/sandbox/sandbox.go	71.06% <0.00%> (-9.90%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/oasis-node/cmd/ias/auth.go	68.96% <0.00%> (-6.90%)	⬇️
...consensus/tendermint/apps/keymanager/keymanager.go	59.34% <0.00%> (-6.60%)	⬇️
go/common/sgx/aesm/aesm.go	63.63% <0.00%> (-6.37%)	⬇️
go/oasis-node/cmd/keymanager/keymanager.go	45.70% <0.00%> (-5.27%)	⬇️
... and 44 more

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