[FEA] Remove deprecated rmm._lib

[bdice]

We recently refactored rmm._lib to rmm.librmm and rmm.pylibrmm, see #1713 and #1676. We deprecated rmm._lib in 24.12, so we can remove rmm._lib in 25.02.

We will need to audit RAPIDS for uses of this private rmm._lib prior to removal but those should be straightforward fixes to imports.

Audit via GitHub search: https://github.com/search?q=org%3Arapidsai%20%22rmm._lib%22&type=code

[bdice]

See also:

rmm/python/rmm/rmm/tests/test_rmm.py

Line 1083 in 73a66c9

# TODO: Remove test when rmm._lib is removed in 25.02