route: remove an outdated comment from rtrequest1_fib

[jacob.e.keller_intel.com]

Summary

revision r334222 modified rtrequest1_fib to always take a reference on
the ifa structure. However, the comment above this indicates the code is
relying on a reference from rt_getifa_fib().

Since this is no longer the case, remove this outdated comment.

Signed-off-by: Jacob Keller <[e-mail]>

[jacob.e.keller_intel.com]

I found this while debugging an extra reference on the ifa refcount. I think I found a "solution" that isn't quite right, but this comment is misleading.