build: Drop ALLOW_HOST_PACKAGES support in depends

The `ALLOW_HOST_PACKAGES` variable was introduced in bitcoin#10508 "to
speed up build and avoid timeout".

It is no longer the case for our CI infrastructure, which uses self-
hosted persistent workers and depends caching.

In the current circumstances, it does not seem worth porting this
feature to the upcoming CMake-based build system.

depends/Makefile

@@ -242,7 +242,6 @@ $(host_prefix)/share/config.site : config.site.in $(host_prefix)/.stamp_$(final_
             -e 's|@CXXFLAGS@|$(strip $(host_CXXFLAGS) $(host_$(release_type)_CXXFLAGS))|' \
             -e 's|@CPPFLAGS@|$(strip $(host_CPPFLAGS) $(host_$(release_type)_CPPFLAGS))|' \
             -e 's|@LDFLAGS@|$(strip $(host_LDFLAGS) $(host_$(release_type)_LDFLAGS))|' \
-            -e 's|@allow_host_packages@|$(ALLOW_HOST_PACKAGES)|' \
             -e 's|@no_qt@|$(NO_QT)|' \
             -e 's|@no_qr@|$(NO_QR)|' \
             -e 's|@no_zmq@|$(NO_ZMQ)|' \