[Issue #1331] Remove jemalloc prefix in pixels-retina

[AntiO2]

Summary

• Remove --with-jemalloc-prefix=je_ from the bundled jemalloc build in cpp/pixels-retina.
• Update JNI jemalloc memory metric calls from je_mallctl to mallctl so they match the unprefixed jemalloc build.
• Keep the optional ENABLE_JEMALLOC=OFF build path compiling.

Validation

• cmake --build cpp/pixels-retina/build --target pixels-retina -j 4: passed.
• jemalloc configure output showed JEMALLOC_PREFIX empty.
• nm -D cpp/pixels-retina/build/jemalloc_install/lib/libjemalloc.so | rg ' (malloc|free|calloc|realloc|mallctl)$| je_(malloc|free|mallctl)$': exported unprefixed malloc, free, calloc, realloc, and mallctl.
• cmake -S cpp/pixels-retina -B /tmp/pixels-retina-no-jemalloc -DENABLE_JEMALLOC=OFF -DFETCHCONTENT_SOURCE_DIR_GOOGLETEST=/home/antio2/projects/pixels/cpp/pixels-retina/build/_deps/googletest-src: passed.
• cmake --build /tmp/pixels-retina-no-jemalloc --target pixels-retina -j 4: passed.

Closes #1331