[MLIR] Drop use of REQUIRES:shell from tests (#168989)
This patch drops two instances of REQUIRES: shell from MLIR tests. This
feature does not mean much given the internal shell is the default for
MLIR. It does prevent these tests from running on Windows, but it does
not seem like there is anything inherent to these tests preventing them
from running on Windows (minus maybe the lack of spirv-tools, which is
explicitly required anyways.
