fix: remove unused x-session-cookie parameter

[aeneasr]

This patch removes the undocumented and experimental X-Session-Cookie header from the /sessions/whoami endpoint.

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #2983 (6e0993a) into master (efb8ae8) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2983      +/-   ##
==========================================
- Coverage   76.17%   76.16%   -0.01%     
==========================================
  Files         308      308              
  Lines       18943    18938       -5     
==========================================
- Hits        14429    14424       -5     
  Misses       3394     3394              
  Partials     1120     1120              

Impacted Files	Coverage Δ
session/manager_http.go	72.38% <100.00%> (-1.00%)	⬇️

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