Use Python implementation of MWU test

Turns out we have a Mann-Whitney U test implementation(for large sample
size) in external.stats.stats directory all along. No longer depends on
SciPy.


git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@208734 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the excrescent argument MWU


git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@208738 91177308-0d34-0410-b5e6-96231b3b80d8