This reverts commit 2d3fa03d53.
Different JDK implementations define JNIEnv differently for C++, so we can't provide the definition here.
Fixes:
/usr/lib/jvm/temurin-11-jdk-amd64/include/jni.h:195:17: error: conflicting declaration ‘typedef struct JNIEnv_ JNIEnv’
/home/runner/work/sdlwiki/sdlwiki/.github/../external/SDL3/include/SDL3/SDL_system.h:266:24: note: previous declaration as ‘typedef struct _JNIEnv JNIEnv’