libxml2/host: revert xml2-config prefix fix

Commit 218f0229a4594b32d9db6ad0678cdce7e2663e27 added a workaround for a
problem. The host compile overwrote xml2-config in
"$(STAGING_DIR)/host/bin". The workaround fixed the issue for regular
target packages. But it didn't leave a proper xml2-config script for
host packages.

Times have changed. Host packages are now installed in
"$(STAGING_DIR_HOSTPKG). So there is no longer any danger of the host
package overwriting the xml2-config script for target packages. So
revert the mentioned commit, leaving us with two proper xml2-config
scripts, one for target package builds and one for host package builds.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>