[improvement] : remove LINODE_CA and use LINODE_CA_BASE64 only

[rahulait]

What this PR does / why we need it:
We set both LINODE_CA and LINODE_CA_BASE64 env vars. When LINODE_CA is set, we set SSL_CERT_DIR env var within the pod as we use x509.SystemCertPool() to set root CAs.
LINODE_CA_BASE64 is used to set CA content in secret used by child clusters as clusterctl doesn't allow us to read from file and set the content.
Instead of having two vars, we can remove one and just rely on LINODE_CA_BASE64.
Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.62%. Comparing base (dba0683) to head (9d5cb0b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #626   +/-   ##
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  Coverage   63.62%   63.62%           
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  Files          70       70           
  Lines        6056     6056           
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  Hits         3853     3853           
  Misses       1946     1946           
  Partials      257      257           

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