Revert "Enable sext.w removal for riscv targets"

This reverts commit bce5507c.

The update of LLVM to clang-r498229 resolved the error with sext.w as
resolved in D143594.

Bug: 275574215
Test: Verified boot of riscv64 phone target.
Change-Id: Ie31b7db03004c5aafb6067713ef5f819e0706b64