test,arch-riscv: Removed the RISCV Insttests

test,arch-riscv: Removed the RISCV Insttests

These tests verify RISCV instructions. This is already one by the
asmtests:
https://gem5.googlesource.com/public/gem5/+/refs/heads/develop/tests/gem5/asmtest/tests.py

The asmtests do this better.

Furthermore, the insttests have some bugs associated with them, which
would require some engineering effort fix:
https://gem5.atlassian.net/browse/GEM5-729
https://gem5.atlassian.net/browse/GEM5-748
https://gem5.atlassian.net/browse/GEM5-749

This patch removes the RISCV insttests.

Change-Id: I9ee3c88d06778823f655ef9222071beb57c6c995
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33147
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>