wimax: drop WiMAX support (bgo #747846)

wimax: drop WiMAX support (bgo #747846)

Even Fedora is no longer shipping the WiMAX SDK, so it's likely we'll
eventually accidentally break some of the code in src/devices/wimax/
(if we haven't already). Discussion on the list showed a consensus for
dropping support for WiMAX.

So, remove the SDK checks from configure.ac, remove the WiMAX device
plugin and associated manager support, and deprecate all the APIs.

For compatibility reasons, it is still possible to create and save
WiMAX connections, to toggle the software WiMAX rfkill state, and to
change the "WIMAX" log level, although none of these have any effect,
since no NMDeviceWimax will ever be created.

nmcli was only compiling in support for most WiMAX operations when NM
as a whole was built with WiMAX support, so that code has been removed
now as well. (It is still possible to use nmcli to create and edit
WiMAX connections, but those connections will never be activatable.)