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=${ANDROID_PLATFORM:-$PLATFORM_VERSION}
+PLATFORM=${ANDROID_PLATFORM:-31}
 BUILD_TOOLS=${ANDROID_BUILD_TOOLS:-31.0.0}
 
 BUILD_DIR="$(realpath ${BUILD_DIR:-build_manual})"