fix(ci): remove pull_request trigger from benchmark workflow

The patrickhuie19/benchmark-action composite action interpolates
${{ github.event.pull_request.body }} directly into a single-quoted
shell string. Any PR body containing a single quote causes:
  unexpected EOF while looking for matching `'

The benchmark workflow's primary value is tracking regressions over
time on main merges. Removing the pull_request trigger eliminates
the breakage without losing that value.

Author: Fletch153

.github/workflows/benchmark.yml

@@ -4,9 +4,6 @@ on:
   push:
     branches:
       - main
-  pull_request:
-    branches:
-      - main
 
 jobs:
   benchmark: