infra: remove separate definition of DISABLE_DOCUMENTATION

Since we now always disable documentation on target, there is no need to
have a separate variable DISABLE_DOCUMENTATION anymore.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>