[trie] Fix duplicate flush and remove flush argument

[Liuhaai]

store() reflush the children by cn.hash(true)

[codecov[bot]]

Codecov Report

Merging #3192 (6543c1b) into master (6da40b6) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3192      +/-   ##
==========================================
+ Coverage   75.13%   75.15%   +0.01%     
==========================================
  Files         221      221              
  Lines       21019    21023       +4     
==========================================
+ Hits        15793    15799       +6     
+ Misses       4391     4389       -2     
  Partials      835      835              

Impacted Files	Coverage Δ
db/trie/mptrie/node.go	100.00% <ø> (ø)
db/trie/mptrie/branchnode.go	98.58% <100.00%> (-0.03%)	:arrow_down:
db/trie/mptrie/cachenode.go	100.00% <100.00%> (ø)
db/trie/mptrie/extensionnode.go	92.98% <100.00%> (ø)
db/trie/mptrie/leafnode.go	100.00% <100.00%> (ø)
db/trie/mptrie/hashnode.go	100.00% <0.00%> (+9.09%)	:arrow_up:

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[raullenchai]

Close b/c it has been inactive for 1+yr.