[RISCV] Remove the TODO that lbu and lhu should be selected to avoid the unnecessary masking. NFC.

RISCVTargetLowering::isZExtFree has been implemented in
llvm@15e894b.

Author: tclin914

llvm/test/CodeGen/RISCV/zext-with-load-is-free.ll

@@ -2,8 +2,6 @@
 ; RUN: llc -mtriple=riscv32 -verify-machineinstrs < %s \
 ; RUN:   | FileCheck %s -check-prefix=RV32I
 
-; TODO: lbu and lhu should be selected to avoid the unnecessary masking.
-
 @bytes = dso_local global [5 x i8] zeroinitializer, align 1
 
 define dso_local i32 @test_zext_i8() nounwind {