Update build.yml to simplify pull_request triggers

Removed pull_request branch specification from build workflow.

Author: vharseko

.github/workflows/build.yml

@@ -4,8 +4,6 @@ on:
   push:
     branches: [ 'sustaining/15.2.x','master' ]
   pull_request:
-    branches: [ 'sustaining/15.2.x','master' ]
-
 jobs:
   build-maven:
     runs-on: ${{ matrix.os }}