refactor(core): remove unnecessary closure in _export()

[cjihrig]

Which problem is this PR solving?

_export() creates an unnecessary closure in order to pass the result of export() to its own resolve() function. This creates unnecessary overhead at runtime.

Fixes # N/A

Short description of the changes

This commit removes the unnecessary closure.

Type of change

Perf refactor

How Has This Been Tested?

• Test suite

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.48%. Comparing base (f15aa7e) to head (4bda627).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6360      +/-   ##
==========================================
- Coverage   95.48%   95.48%   -0.01%     
==========================================
  Files         363      363              
  Lines       11564    11563       -1     
  Branches     2669     2669              
==========================================
- Hits        11042    11041       -1     
  Misses        522      522              

Files with missing lines	Coverage Δ
...ckages/opentelemetry-core/src/internal/exporter.ts	100.00% <100.00%> (ø)

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[otelbot]

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