ctfdump: Remove definitions of warn() and vwarn()

The presence of the latter causes a link error when building a
statically linked ctfdump(1) because libc defines the same symbol.
libc's warn() is defined as a weak symbol and so does not cause the same
problem, but let's just use libc's version.

Reported by: stephane rochoy <stephane.rochoy@stormshield.eu>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation