[RISCV] Remove NotHasStdExtZbb predicate from zext.h/sext.b/sext.h InstAliases. NFC

NotHasStdExtZbb doesn't have an AssemblerPredicate associated with it
so it didn't do anything. We don't need it either because the sorting
rules in tablegen prioritize by number of predicates. So the
dedicated instructions in the B extension that have predicates
will be prioritized automatically.