devel/llvm*: Remove unnecessary C/CFLAGS definitions

As of 3c5189a0fd4c82addfb659024bf28c5006bf88f7
there's no need to define DNDEBUG manually

PR: 277909
Approved by: portmgr (maintainer timeout, 2+ weeks)

(cherry picked from commit 1f50c8e24264be1d64a96e84536e2e1d59b63180)