chore: remove JSC from the build matrix (#2754)

## Summary:

For React Native 0.81+ (the next version we're syncing to), JSC has been
removed from the repo and must be installed via. third party package
(see
https://reactnative.dev/blog/2025/08/12/react-native-0.81#community-maintained-javascriptcore-support
). As such, let's remove JSC from our build matrix.

## Test Plan:

CI should pass

Author: Saadnajmi

.github/workflows/microsoft-build-rntester.yml

@@ -5,15 +5,14 @@ on:
 
 jobs:
   build-rntester:
-    name: "${{ matrix.platform }}, ${{ matrix.arch }}, ${{ matrix.engine }}"
+    name: "${{ matrix.platform }}, ${{ matrix.arch }}"
     runs-on: macos-26
     timeout-minutes: 90
     strategy:
       fail-fast: false
       matrix:
         platform: [macos, ios, visionos]
         arch: [oldarch, newarch]
-        engine: [jsc, hermes]
         include:
           # Platform-specific properties
           - platform: macos
@@ -33,10 +32,6 @@ jobs:
             new_arch_enabled: '0'
           - arch: newarch
             new_arch_enabled: '1'
-          - engine: jsc
-            use_hermes: '0'
-          - engine: hermes
-            use_hermes: '1'
 
     steps:
       - uses: actions/checkout@v4
@@ -58,7 +53,6 @@ jobs:
         working-directory: packages/rn-tester
         env:
           RCT_NEW_ARCH_ENABLED: ${{ matrix.new_arch_enabled }}
-          USE_HERMES: ${{ matrix.use_hermes }}
         run: |
           set -eox pipefail
           bundle install