Check if legacy offline queue file exists

[gandarez]

This PR avoids trying to delete a non existing legacy offline queue file.

failed to delete legacy offline file: remove <path>/.wakatime.bdb: no such file or directory

[codecov]

Codecov Report

Attention: Patch coverage is 58.82353% with 14 lines in your changes missing coverage. Please review.

Project coverage is 63.00%. Comparing base (65ec92a) to head (65d83f0).

@@             Coverage Diff             @@
##           develop    #1079      +/-   ##
===========================================
- Coverage    63.05%   63.00%   -0.05%     
===========================================
  Files          383      383              
  Lines        16504    16500       -4     
===========================================
- Hits         10406    10396      -10     
- Misses        5531     5537       +6     
  Partials       567      567              

Flag	Coverage Δ
unittests	63.00% <58.82%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
cmd/offline/offline.go	73.64% <ø> (-0.53%)	⬇️
cmd/offlinecount/offlinecount.go	37.50% <100.00%> (-14.89%)	⬇️
cmd/offlineprint/offlineprint.go	53.33% <100.00%> (-4.25%)	⬇️
cmd/offlinesync/offlinesync.go	56.07% <100.00%> (+2.80%)	⬆️
cmd/params/params.go	85.96% <100.00%> (-0.09%)	⬇️
cmd/heartbeat/heartbeat.go	77.91% <0.00%> (+1.01%)	⬆️
cmd/run.go	42.43% <0.00%> (ø)
pkg/offline/legacy.go	30.76% <25.00%> (-19.24%)	⬇️
pkg/offline/offline.go	55.74% <25.00%> (-0.83%)	⬇️