riscv: kselftests: Fix mm build by removing testcases subdirectory

kselftests fails to build because the mm/testcases subdirectory is not
created and then the compiler fails to output the binary there.

So fix this by simply removing this subdirectory which is not very
useful.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20230915100113.13131-1-alexghiti@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>