asterisk-16.x: remove libxml2 hacks

asterisk-16.x: remove libxml2 hacks

There were problems in the past with OpenWrt's libxml2 package (also
it's host package). This led to quite a few workarounds in the asterisk
Makefile.

Recent changes in the libxml2 package have cleared this up.

1. a proper xml2-config file is now installed by both libxml2 and
   libxml2/host
2. xml2-config is now installed with a host triplet prefix (e.g.
   "mips-openwrt-linux-xml2-config" for libxml2 and
   "x86_64-pc-linux-gnu-xml2-config" for libxml2/host), additionally
   these tools get a symbolic link to xml2-config

That means that now OpenWrt's libxml2 can be properly detected and the
workarounds are no longer required.

This commit also removes an explicit dependency on libxml2 from
res-calendar-caldav. The base package already depends on libxml2, hence
the dependency is implicit.

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