Revert "common.sh: force parallel=16 to avoid exhausting memory when running...

Revert "common.sh: force parallel=16 to avoid exhausting memory when running 48 parallel build jobs"

This reverts commit 47571d6f.

Limiting the number of parallel jobs was mainly necessary for being able
to build meson and imath. Now that gstreamer 1.24 is uploaded, we don't
need any of that anymore and building linux with 48 works just fine.