core/rawdb: don't warn for missing "unclean shutdown markers"

[ucwong]

This changes removes the warn-printout about not finding unclean shutdown markers, which always happens on fresh installs / wiped databases.

[codecov]

Codecov Report

Merging #1711 (eddc228) into master (3af42fe) will decrease coverage by 0.07%.
Report is 4 commits behind head on master.
The diff coverage is 10.00%.

❗ Current head eddc228 differs from pull request most recent head 9763344. Consider uploading reports for the commit 9763344 to get more accurate results

@@            Coverage Diff             @@
##           master    #1711      +/-   ##
==========================================
- Coverage   52.69%   52.63%   -0.07%     
==========================================
  Files         484      484              
  Lines       60638    60637       -1     
==========================================
- Hits        31955    31918      -37     
- Misses      26251    26292      +41     
+ Partials     2432     2427       -5     

Files Changed	Coverage Δ
cmd/devp2p/nodesetcmd.go	0.00% <0.00%> (ø)
core/rawdb/accessors_metadata.go	0.00% <0.00%> (ø)
ctxc/backend.go	0.00% <0.00%> (ø)
ctxc/discovery.go	0.00% <0.00%> (ø)
ctxc/handler.go	37.87% <100.00%> (ø)

... and 14 files with indirect coverage changes