Remove useless intermediate variable

Now that PLATFORM is correctly used in the if-condition,
PLATFORM_VERSION is useless.

PR #2850 <https://github.com/Genymobile/scrcpy/pull/2850>

server/build_without_gradle.sh

@@ -14,8 +14,7 @@ set -e
 SCRCPY_DEBUG=false
 SCRCPY_VERSION_NAME=1.21
 
-PLATFORM_VERSION=31
+PLATFORM=${ANDROID_PLATFORM:-31}
-PLATFORM=${ANDROID_PLATFORM:-$PLATFORM_VERSION}
 BUILD_TOOLS=${ANDROID_BUILD_TOOLS:-31.0.0}
 
 BUILD_DIR="$(realpath ${BUILD_DIR:-build_manual})"
@@ -57,7 +56,7 @@ javac -bootclasspath "$ANDROID_JAR" -cp "$CLASSES_DIR" -d "$CLASSES_DIR" \
 echo "Dexing..."
 cd "$CLASSES_DIR"
 
-if [[ $PLATFORM_VERSION -lt 31 ]]
+if [[ $PLATFORM -lt 31 ]]
 then
     # use dx
     "$ANDROID_HOME/build-tools/$BUILD_TOOLS/dx" --dex \