Remove idempotency note about push rule endpoint

What does this MR do and why?

Contributes to #427352 (closed)

See also thread: #427352 (comment 1647445610)

Problem

HTTP DELETE verb is idempotent by default. When users call it multiple times, they will get different responses but the result will be the same - the push rule is deleted.

But the same thing applies to other DELETE endponts. There is no actual reason to specify it here.

Solution

Update documentation and remove the mention of idempotency from push rule delete endpoint.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.