Fix build for RPM 4.20

With earlier versions, this points to the global BUILDROOT directory, which
is already empty at this stage.  However, RPM 4.20 rearranged the working
directories (which are supposed to be internal implementation details) and
this ends up removing the unpacked source tree instead and failing later.

open-vm-tools.spec

@@ -227,8 +227,6 @@ machines.
 %build
 %if 0%{?bundle_gtkmm3}
 export PATH="%{_buildrootdir}/bin:$PATH"
-# Cleanup buildroot for existing rpms
-rm -rf %{_buildrootdir}/*
 
 function install_rpms_to_current_dir() {
     PACKAGE_RPM=$(eval echo $1)