CMake: Fix build on Android NDK

Android NDK does not have executables in the format of
aarch64-linux-android-clang++. Instead it provides executables in the
format aarch64-linux-android${ANDROID_ABI}-clang++. Since CMake 3.21 all
of this is handled by the toolchain file within Android NDK itself, so
there is no need to override the compilers anymore.

cmake/platforms/Android-aarch64-clang.cmake

@@ -4,10 +4,6 @@
 
 include(${GEOGRAM_SOURCE_DIR}/cmake/platforms/Linux.cmake)
 
-# Set the Android compilers
-set(CMAKE_CXX_COMPILER aarch64-linux-android-clang++)
-set(CMAKE_C_COMPILER aarch64-linux-android-clang)
-
 set(VORPALINE_ARCH_64 true)
 
 # No graphics (yet) for Android