Remove unused parameter

[rdwebdesign]

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• What does this PR aim to accomplish?:

  The parameter $error_on_failure wasn't used inside pihole_execute function.

• How does this PR accomplish the above?:

  Remove unused parameter from the function.

• What documentation changes (if any) are needed to support this PR?:

  none

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[yubiuser]

Added by #1141 but never used

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-16-web-v5-13-and-core-v5-11-1-released/56384/1