Remove EventType Key in code

[PravinPK]

Description

Remove the unwanted eventType key from constants file

Related Issue

Motivation and Context

This key seems to be confusing as eventType field is not mandatory for Experience edge events

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #129 (759bbfa) into main (2519205) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #129   +/-   ##
=========================================
  Coverage     84.30%   84.30%           
  Complexity      406      406           
=========================================
  Files            30       30           
  Lines          1643     1643           
  Branches        235      235           
=========================================
  Hits           1385     1385           
  Misses          165      165           
  Partials         93       93           

Flag	Coverage Δ
functional-tests	67.07% <ø> (ø)
unit-tests	80.34% <ø> (ø)

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[kevinlind]

The eventType field is not required to be set by customer, but we do recommend setting it. We set an eventType when creating internal experience events, but that is done in other extensions, so it looks like this variable isn't needed.
Can you also clean up the other XDM variables as mentioned in the comments? It looks like EVENT_ID, TIMESTAMP, and IDENTITY_MAP are the only ones used in production.