vm_radix: use bool, drop function

[dougm]

Summary

Replace boolean_t with bool in vm_radix.c. Drop the unused function vm_radix_is_singleton, which is unused and has no corresponding function in subr_pctrie.c.

Test Plan

Kernel builds and boots.

[dougm]

Update line numbers.

[dougm]

Checked it in. Didn't automatically get closed because I use <angle brackets>.