Remove _type field from index and delete commands

[rowanseymour]

@ericnewcomer @norkans7 I've tested locally running this (with #70) on an existing index created with types and there doesn't seem to be any problem.

Removal of types within indexes (we don't use multi-type indexes) is the big breaking change between ES7/OS1 and ES8/OS2.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.96%. Comparing base (c4f3ec9) to head (9e8f40d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #71   +/-   ##
=======================================
  Coverage   50.96%   50.96%           
=======================================
  Files           6        6           
  Lines         569      569           
=======================================
  Hits          290      290           
  Misses        254      254           
  Partials       25       25           

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[norkans7]

@rowanseymour we need to update mailroom about that as well

[rowanseymour]

@norkans7 just checked and the search commands we issue from mailroom don't use _type - the client library has a Type(string) method but it even mentions that this is deprecated and we don't use it.