build: fix mingw rpm build

build: fix mingw rpm build

Commit d13155c changed which files get installed for the
libvirt-guests service, but did not touch up the mingw spec
file.  As a result, rpmbuild complained:

RPM build errors:
    Installed (but unpackaged) file(s) found:
   /usr/i686-w64-mingw32/sys-root/mingw/libexec/libvirt-guests.sh
   /usr/x86_64-w64-mingw32/sys-root/mingw/libexec/libvirt-guests.sh

which in turn breaks ./autobuild.sh when cross-compilers are present.

* mingw-libvirt.spec.in: Remove files not relevant to mingw.