Remove INTERFACE build targets.

Commit b04f3b9a (Create make rules for INTERFACE_LIBRARY
targets., 2013-08-21) extended the makefile generator to create
build targets for INTERFACE_LIBRARY targets. No other generators
were extended with this feature.

This conflicts with the feature of whitelisting of target properties
read from INTERFACE_LIBRARY targets. The INTERFACE_* properties
of the INTERFACE_LIBRARY may legitimately contain TARGET_PROPERTY
generator expressions for reading properties from the 'head target'.
The 'head target' would be the INTERFACE_LIBRARY itself when creating
the build rules for it, which means that non-whitelisted properties
would be read.

Tests/InterfaceLibrary/dummy.cpp

@@ -0,0 +1,5 @@
+
+int main(int, char **)
+{
+  return 0;
+}