Remove COMPILER from Makefile

The same functionality is available by using COMPCXX and having another variable which does the same is just confusing.
There was only one mention on Stockfish Wiki about this which has been changed to COMPCXX.

closes https://github.com/official-stockfish/Stockfish/pull/5154

No functional change

.github/workflows/tests.yml

@@ -6,7 +6,7 @@ jobs:
     name: ${{ matrix.config.name }}
     runs-on: ${{ matrix.config.os }}
     env:
-      COMPILER: ${{ matrix.config.compiler }}
+      COMPCXX: ${{ matrix.config.compiler }}
       COMP: ${{ matrix.config.comp }}
       CXXFLAGS: "-Werror"
     strategy:
@@ -172,9 +172,9 @@ jobs:
             if [ $COMP == ndk ]; then
               export PATH=${{ env.ANDROID_NDK_BIN }}:$PATH
             fi
-            $COMPILER -v
+            $COMPCXX -v
           else
-            echo "$COMPILER -v" > script.sh
+            echo "$COMPCXX -v" > script.sh
             docker run --rm --platform ${{ matrix.config.platform }} -v ${{ github.workspace }}/src:/app sf_builder
           fi