iTerm password manager sends \x03\0x7f before sending a password
from its password manager to make sure the password is not being
echoed to the screen.  Unfortunately, vault login does not handle
the Space DEL sequence, causing the login to fail when using the
password manager.  This patch uses a simple method to delete the
sequence if present anywhere in the string, although it is strictly
only needed at the start of input.

The logic now only removes the two byte prefix sent in by iTerm
instead of trying to remove all deletes in the string.

This has been tested to work with the iTerm password manager.

As a small correction, the byte sequence is \x20\x7f.  The
earlier commit message incorrectly stated it was \x03\x7f.