riscv: tlbflush: remove confusing comment on local_flush_tlb_all()

Remove a confusing comment on our local_flush_tlb_all()
implementation.  Per an internal discussion with Andrew, while it's
true that the fence.i is not necessary, it's not the case that an
sfence.vma implies a fence.i.  We also drop the section about
"flush[ing] the entire local TLB" to better align with the language in
section 4.2.1 "Supervisor Memory-Management Fence Instruction" of the
RISC-V Privileged Specification v20190608.

Fixes: c901e45a ("RISC-V: `sfence.vma` orderes the instruction cache")
Reported-by: Alan Kao <alankao@andestech.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Andrew Waterman <andrew@sifive.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>