Deprecate OpenCensus-based internal data structures

[tigrannajaryan]

All new code must use the new pdata.* structures.

TODO: move a few remaining functions that translate between
OpenCensus-data and pdata to OpenCensus packages.

[codecov]

Codecov Report

Merging #1843 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1843   +/-   ##
=======================================
  Coverage   91.16%   91.16%           
=======================================
  Files         271      271           
  Lines       16235    16235           
=======================================
  Hits        14801    14801           
  Misses       1008     1008           
  Partials      426      426           

Impacted Files	Coverage Δ
translator/internaldata/metrics_to_oc.go	91.17% <ø> (ø)
translator/internaldata/oc_to_metrics.go	91.97% <ø> (ø)
translator/internaldata/oc_to_traces.go	85.46% <ø> (ø)
translator/internaldata/traces_to_oc.go	86.53% <100.00%> (ø)

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