Deactivate the mark.
If Transient Mark mode is disabled, this function normally does
nothing; but if FORCE is non-nil, it deactivates the mark anyway.
Deactivating the mark sets `mark-active' to nil, updates the
primary selection according to `select-active-regions', and runs
`deactivate-mark-hook'.
If Transient Mark mode was temporarily enabled, reset the value
of the variable `transient-mark-mode'; if this causes Transient
Mark mode to be disabled, don't change `mark-active' to nil or
run `deactivate-mark-hook'.
If an editing command sets this to t, deactivate the mark afterward.
The command loop sets this to nil before each command,
and tests the value when the command returns.
Buffer modification stores t in this variable.