elf_common.h: remove obsolete riscv relocations

These were specified at one time, but later removed.

Reviewed by:	jrtc27, imp, emaste
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D38174

sys/sys/elf_common.h

@@ -1364,8 +1364,6 @@ typedef struct {
 #define	R_RISCV_SUB16		38
 #define	R_RISCV_SUB32		39
 #define	R_RISCV_SUB64		40
-#define	R_RISCV_GNU_VTINHERIT	41
-#define	R_RISCV_GNU_VTENTRY	42
 #define	R_RISCV_ALIGN		43
 #define	R_RISCV_RVC_BRANCH	44
 #define	R_RISCV_RVC_JUMP	45