Fix SPIFFE url handling

[andrewstucki]

Changes proposed in this PR:

When users use root connect certificates without SPIFFE information (i.e. via Vault), the controller crashes. This removes the piece of code in our mTLS middleware that verifies the corresponding host between root cert and the derived leaf cert. This should be alright since we're still validating that the root actually generated the leaf certificate cryptographically before these checks.

Fixes: #208

Checklist:

• CHANGELOG entry added

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