Remove duplicate is_lexsorted function
`_libs.lib` and `_libs.algos` have near-identical `is_lexsorted` functions.  The only differences appear to be small optimizations/modernizations in the `algos` version.  AFAICT the `algos` version is only used in `tests.test_algos` ATM.  This PR removes the `libs._lib` version and changes the one usage (in `indexes.multi`) to use the `algos` version.
