This property was left from before CMake always linked using full path
library names for targets it builds. In order to safely link with
"-lfoo" we needed to avoid having both shared and static libraries in
the build tree for targets that switch on BUILD_SHARED_LIBS. This meant
cleaning both shared and static names before creating the library, which
led to the creation of CLEAN_DIRECT_OUTPUT to disable the behavior.
Now that we always link with a full path we do not need to clean old
library names left from an alternate setting of BUILD_SHARED_LIBS. This
change removes the CLEAN_DIRECT_OUTPUT property and instead uses its
behavior always. It removes some complexity from cmTarget internally.
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