sys/conf: Remove an unneeded flag variable

After commit fac6dee9eb58 ("Remove tests for obsolete compilers in the
build system"), we always set -fdebug-prefix-map, so there's no point in
defining and testing _MAP_DEBUG_PREFIX. No functional change intended.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ab8b2d108c7c4e63ed850148fdc237633077fcd3)