build: fix overlinking to libdw

build: fix overlinking to libdw

We have two AC_CHECK_LIB checks for libdw. The first one has an empty
action-if-found, so it defaults to adding "-ldw" to LIBS. LIBS are
applied to everything we build. But only systemd-coredump needs libdw.
It already links to ELFUTILS_LIBS correctly in Makefile.am.

Drop the first AC_CHECK_LIB check. The second check is sufficient.

Q: Don't we already use "-Wl,--as-needed" to eliminate overlinking?
A: We do, but it is effective only for executables, not for shared
   libraries. This is due to a libtool bug:
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=347650