Remove qiskit-bot notification rule for rust changes (#13198)

We previously had a qiskit-bot notification trigger for changes to the
rust source or cargo configuration that would list myself and
`@kevinhartman` as the relevant reviewers. While this is still true, as
Qiskit is now >16% Rust (by lines of code, excluding tests) and that
ratio will only grow over time, the two of us are hardly the only people
worth calling out specifically as reviewers of rust code. Instead of
expanding the list to include the entirety of the terra-core team
individually this just removes the custom notification rule and opts to
fallback to the default terra-core entry.

qiskit_bot.yaml

@@ -25,9 +25,6 @@ notifications:
         - "`@t-imamichi`"
         - "`@ajavadia`"
         - "`@levbishop`"
-    ".*\\.rs$|^Cargo":
-        - "`@mtreinish`"
-        - "`@kevinhartman`"
     "(?!.*pulse.*)\\bvisualization\\b":
         - "@enavarro51"
     "^docs/":