In CMake 3.6 and below, running

    cmake --build . --target "$(pwd)/SomeTarget"

with a Makefiles generator automatically converted the target
name and invoked `make SomeTarget`.  This made the build command
work even though

    make "$(pwd)/SomeTarget"

would fail.  This behavior was not implemented for any other generators,
and does not make sense because `cmake --build` is supposed to be a thin
wrapper around the native build tool.  It has also been broken since
commit 8d47a20f (cmOutputConverter: use new ConvertToRelativePath
signature internally, 2016-06-16, v3.7.0-rc1~90^2~1) because cmState's
relative path conversion logic is not initialized in `cmake --build`.

Remove the non-functioning code.