[Fuchsia] Remove aarch64-fuchsia target-specific -mcmodel=kernel

Under --target=aarch64-fuchsia, -mcmodel=kernel has the effect of
(the default) -mcmodel=small plus -mtp=el1 (which did not exist when
this behavior was added). Fuchsia's kernel now uses -mtp=el1
directly instead of -mcmodel=kernel, so remove this special support.

Patch By: mcgrathr

Differential Revision: https://reviews.llvm.org/D73409