rt: Remove some duplicated code from 64-bit __morestack

Author: brson

src/rt/arch/x86_64/morestack.S

@@ -71,18 +71,6 @@ MORESTACK:
 	leaq 16(%rbp), %rax
 	pushq %rax
 
-	// During unwinding we want to skip our caller since it's not
-	// a complete frame and will make the unwinder sad
-	// Don't understand this line
-	.cfi_offset 16, 0
-	// Tell the unwinding where to get the stack pointer for
-	// our grandparent frame
-	.cfi_offset %rsp, -24
-
-	// Save the grandparent stack pointer for the unwinder
-	leaq 16(%rbp), %rax
-	pushq %rax
-
 	// FIXME: libgcc also saves rax. not sure if we need to
 
 	// Save argument registers