riscv: vdso.lds.S: drop __alt_start and __alt_end symbols

These two symbols are not used, remove them.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Tested-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://lore.kernel.org/r/20230912072015.2424-2-jszhang@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/vdso/vdso.lds.S

@@ -42,9 +42,7 @@ SECTIONS
 
 	. = ALIGN(4);
 	.alternative : {
-		__alt_start = .;
 		*(.alternative)
-		__alt_end = .;
 	}
 
 	.data		: {