Remove import of kahypar after all

In commit eb98361 I added the import of kahypar, which (at least at the time) appeared to have been imported by Quimb. Double-checking this import in clean environments reveals that in fact, nothing depends on kahypar.

Taking it out via a separate commit because right now this package is causing our GitHub actions for commit checks to fail, and I want to leave a record of what caused the failures and how they were resolved.

Author: mhucka

cirq-core/cirq/contrib/requirements.txt

@@ -6,4 +6,3 @@ pylatex~=1.4
 # quimb
 quimb>=1.8
 opt_einsum
-kahypar