fix(exporter): Don't lock for non bulk exporter

[thomaspoignant]

Description

In this PR we stop using the slice storing the events for non-bulk exporter.
This is not needed because we call Export directly when we receive a featureEvent and it was causing delay when waiting on the Lock function of the mutex.

Closes issue(s)

Resolve #2792

Checklist

• I have tested this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

[netlify]

✅ Deploy Preview for go-feature-flag-doc-preview canceled.

Name	Link
🔨 Latest commit	149dec3
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/675c3c09b8c62000081ba760

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

Attention: Patch coverage is 48.57143% with 18 lines in your changes missing coverage. Please review.

Project coverage is 84.77%. Comparing base (9a9dbce) to head (149dec3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
exporter/data_exporter.go	48.57%	17 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2793      +/-   ##
==========================================
- Coverage   84.83%   84.77%   -0.06%     
==========================================
  Files         111      111              
  Lines        5184     5190       +6     
==========================================
+ Hits         4398     4400       +2     
- Misses        623      626       +3     
- Partials      163      164       +1     

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