analyzer: fix "unused variable 'summary_cast_reg'" warning

I missed this in r15-1108-g70f26314b62e2d.

gcc/analyzer/ChangeLog:
	* call-summary.cc
	(call_summary_replay::convert_region_from_summary_1): Drop unused
	local "summary_cast_reg"

Signed-off-by: David Malcolm <dmalcolm@redhat.com>