two-step: Remove ':' at the end of (passphrase) prompt below text entry field

two-step: Remove ':' at the end of (passphrase) prompt below text entry field

When asked for e.g. a disk unlock passphrase, plymouth will be passed
a prompt like: "Please enter passphrase for disk $DISK:" the ':' in
the end makes sense when asking for this a text console, but this makes
less sense in the two-step disk unlock screen where the text is below
the passphrase entry field.

Strip any ':' char at the prompt's end on two-step's disk unlock screen.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2356893
Signed-off-by: Hans de Goede <hansg@kernel.org>