RISC-V: remove the unused return_to_handler export

This export is not only not needed, but also breaks symbol versioning
due to being an undeclared assembly export.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/mcount.S

@@ -75,7 +75,6 @@ ENTRY(return_to_handler)
 	RESTORE_RET_ABI_STATE
 	jalr	a1
 ENDPROC(return_to_handler)
-EXPORT_SYMBOL(return_to_handler)
 #endif
 
 #ifndef CONFIG_DYNAMIC_FTRACE