Fix gold testsuite failure with GCC 6.

With GCC 6 when not using -static-libstdc++, the operator delete(void*)
function is defined in the shared C++ support library, rather than in
the main program. The test script is too aggressive in checking for
this symbol's presence among the exported symbols. This patch removes
the check for that symbol.

gold/
	PR gold/20310
	* testsuite/dynamic_list.sh: Remove check for _ZdlPv.