Follow-up r365662 (MFC of r365371 and r365373) by also removing the
header hack from jemalloc_FreeBSD.h, which rendered any make.conf
MALLOC_PRODUCTION or src.conf WITH/WITHOUT_MALLOC_PRODUCTION irrelevant.

Direct commit to stable/{11,12} as this does not apply to head.

Noticed by: kevans

- Remove debugging from GENERIC* kernel configurations
- Enable MALLOC_PRODUCTION
- Default dumpdev=NO
- Remove UPDATING entry regarding debugging features

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation