msys-core: Remove the path which add Cygwin's version of kill

Now that we have

https://github.com/git-for-windows/git/commit/d0243ad1a59ef58b8711fe124f0c6a9a8abce86c

even applied upstream, see

https://groups.google.com/d/msg/msysgit/76LDghCMU8k/x8uRgof-i4wJ

there is no need to ship Cygwin's kill anymore. This fixes the "probable
package conflict; existing file not overwritten" error when extracting
"kill.exe", see

https://dscho.cloudapp.net/job/sdk-test-installer/61/artifact/mingw-get.log

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>