rpc: remove Cause field from RPC error

[AnnaShaleva]

We can use Data field for the same purposes.

[codecov]

Codecov Report

Merging #2544 (ea6ddbe) into master (7dcbfcb) will decrease coverage by 0.00%.
The diff coverage is 58.53%.

@@            Coverage Diff             @@
##           master    #2544      +/-   ##
==========================================
- Coverage   85.01%   85.00%   -0.01%     
==========================================
  Files         295      296       +1     
  Lines       37144    37175      +31     
==========================================
+ Hits        31578    31602      +24     
- Misses       4219     4226       +7     
  Partials     1347     1347              

Impacted Files	Coverage Δ
pkg/rpc/server/server.go	77.51% <52.21%> (+0.09%)	⬆️
pkg/rpc/response/errors.go	69.69% <64.28%> (-13.07%)	⬇️
pkg/rpc/server/error.go	82.60% <82.60%> (ø)
pkg/core/interop/runtime/engine.go	95.52% <0.00%> (-4.48%)	⬇️
pkg/core/interop/storage/find.go	97.33% <0.00%> (-2.67%)	⬇️
pkg/core/native/ledger.go	93.16% <0.00%> (-0.86%)	⬇️
pkg/core/dao/dao.go	80.10% <0.00%> (-0.84%)	⬇️
pkg/vm/contract_checks.go	81.29% <0.00%> (-0.53%)	⬇️
pkg/core/interop/context.go	92.03% <0.00%> (-0.11%)	⬇️
pkg/network/server.go	74.11% <0.00%> (-0.10%)	⬇️
... and 12 more

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