ld: Remove unused expression state
    
    Previous commit removed all uses of the defsym field within the linker
    expression union.  This commit cleans up the now redundant state.
    
    ld/ChangeLog:
    
    	* ldexp.h (union etree_union): Remove defsym field.
    	* ldexp.c (exp_assop): Remove defsym parameter, and use of defsym
    	parameter.
    	(exp_assign): Remove passing of defsym parameter.
    	(exp_defsym): Likewise.
    	(exp_provide): Likewise.