remove ioutils

[juniuszhou]

According to Halborn, the ioutil package was out of date, we could use both os and io to replace it.

[codecov]

Codecov Report

Merging #3243 (e6d7648) into future/peggy2 (398e4dd) will not change coverage.
The diff coverage is 14.28%.

@@              Coverage Diff               @@
##           future/peggy2    #3243   +/-   ##
==============================================
  Coverage          47.13%   47.13%           
==============================================
  Files                147      147           
  Lines              12474    12474           
==============================================
  Hits                5880     5880           
  Misses              6238     6238           
  Partials             356      356           

Impacted Files	Coverage Δ
app/export.go	11.84% <0.00%> (ø)
...er/internal/symbol_translator/symbol_translator.go	75.00% <0.00%> (ø)
cmd/sifgen/main.go	0.00% <0.00%> (ø)
tools/sifgen/network/network.go	0.00% <0.00%> (ø)
tools/sifgen/node/node.go	0.00% <0.00%> (ø)
tools/sifgen/sifgen.go	0.00% <0.00%> (ø)
tools/sifgen/utils/cli.go	23.37% <0.00%> (ø)
x/dispensation/utils/parser.go	53.84% <100.00%> (ø)
x/tokenregistry/utils/parse_denom_list.go	40.00% <100.00%> (ø)