Revert "Add ability to disable multihead support"

The variable is set not from config nor anywhere else when kwin is
started through ksmserver by startkde.

In addition the KGlobal implementation is twisted compared to the kcminit
config behavior (the config value defaults to true, not false - ie. if the
variable isn't set (by kcm init) it's reasonable to assume true either.

Therefore and in alignment with PW/2, the environment is read directly and
on absence resolved to "true".

To control the behavior, please export KDE_MULTIHEAD=true/false before starting KWin
(eg. in /usr/bin/startkde)

This reverts commit ab6d5c048a25bcb2f5bdb822ba3eda64019c61bc.

REVIEW: 112579