runtime(less): reset readonly setting when disabling less mode

runtime(less): reset readonly setting when disabling less mode

While at it, also do the unmap only if a specific key has been mapped.
There are some keys that are only selectively mapped and it would cause
an error if we are trying to unmap such a key (e.g. z when your
foldmethod is not manual).

fixes: #14040

Signed-off-by: Christian Brabandt <cb@256bit.org>