exporter/signalfx: Use updated network metric name

[asuresh4]

Description: Update usages of system.network.dropped_packets to system.network.dropped as a result of open-telemetry/opentelemetry-collector#2311.

The occurrence in translation/constants.go is being removed here #2161

[codecov]

Codecov Report

Merging #2160 (32bedb4) into master (9f137ea) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2160      +/-   ##
==========================================
+ Coverage   90.37%   90.40%   +0.03%     
==========================================
  Files         397      397              
  Lines       19506    19506              
==========================================
+ Hits        17628    17635       +7     
+ Misses       1416     1406      -10     
- Partials      462      465       +3     

Flag	Coverage Δ
integration	69.75% <ø> (+0.06%)	⬆️
unit	89.19% <ø> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
processor/groupbytraceprocessor/event.go	96.00% <0.00%> (-0.80%)	⬇️
receiver/carbonreceiver/transport/tcp_server.go	67.00% <0.00%> (+1.00%)	⬆️
receiver/prometheusexecreceiver/receiver.go	88.33% <0.00%> (+2.50%)	⬆️
receiver/redisreceiver/interval/interval_runner.go	42.10% <0.00%> (+21.05%)	⬆️

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