Remove control+r handling from geliboot's pwgets()

pwgets() is based on ngets() from libstand, which includes a feature
that is not wanted in a very of the function designed for password
handling.

Pressing control+r echos out the entered string

This commit removes that feature from pwgets()

PR: 217298
Reported by: ehaupt
Reviewed by: kristof, tsoome, ehaupt
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D9782