rpc: marshal Asset hash in LE in getnep5balances RPC

[fyrchik]

Asset hash should be returned without leading 0x in little-endian.

[codecov]

Codecov Report

Merging #751 into master will decrease coverage by 0.08%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #751      +/-   ##
=========================================
- Coverage   64.78%   64.7%   -0.09%     
=========================================
  Files         139     140       +1     
  Lines       12734   12751      +17     
=========================================
  Hits         8250    8250              
- Misses       4083    4100      +17     
  Partials      401     401

Impacted Files	Coverage Δ
pkg/rpc/response/result/nep5.go	0% <0%> (ø)

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