refactor(xignite): remove unnecessary comments and files

[dakimura]

WHAT

• remove unnecessary comment like //-------
• change level of the log shown when ListIndexSymbols API fails from WARN to ERROR
• add Info log before calling GetRealTimeQuotes API

WHY

• refactor

[codecov-commenter]

Codecov Report

Merging #531 (07d86ef) into master (0f59ea9) will decrease coverage by 0.08%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #531      +/-   ##
==========================================
- Coverage   48.05%   47.96%   -0.09%     
==========================================
  Files         125      125              
  Lines       12375    12375              
==========================================
- Hits         5947     5936      -11     
- Misses       5931     5942      +11     
  Partials      497      497              

Impacted Files	Coverage Δ
contrib/xignitefeeder/api/model.go	0.00% <ø> (ø)
contrib/xignitefeeder/configs/config.go	49.23% <ø> (ø)
contrib/xignitefeeder/symbols/manager.go	23.25% <0.00%> (ø)
contrib/xignitefeeder/writer/quotes_writer.go	92.85% <ø> (ø)
contrib/xignitefeeder/api/client.go	61.15% <100.00%> (ø)
frontend/stream/stream.go	58.97% <0.00%> (-12.83%)	⬇️
metrics/du.go	90.90% <0.00%> (+18.18%)	⬆️