makefiles/buildtest: always execute 'buildtest' loop on host machine

This remove executing buildtest `for` loop in docker.

When building completely in docker, 'buildtest' would hide issues when
the host toolchain would be used when doing `make all` directly.
It has the consequence that it now starts a container for each
compilation which is slower.

The previous behavior can be reproduced by using

    BUILD_IN_DOCKER=1 make buildtest-indocker

A side effect is also that now `BUILDTEST_MAKE_REDIRECT` would work when
doing `buildtest` with docker.

makefiles/buildtests.inc.mk

@@ -2,9 +2,6 @@
 
 BUILDTEST_MAKE_REDIRECT ?= >/dev/null 2>&1
 
-ifeq ($(BUILD_IN_DOCKER),1)
-buildtest: ..in-docker-container
-else
 buildtest:
 	@ \
 	RESULT=true ; \
@@ -24,7 +21,6 @@ buildtest:
 		fi; \
 	done ; \
 	$${RESULT}
-endif # BUILD_IN_DOCKER
 
 # Define 'buildtest-indocker' completely executed inside the container.
 # It prevents starting one container per compilation wich is slower but it

makefiles/docker.inc.mk

@@ -5,7 +5,6 @@ export DOCKER_FLAGS ?= --rm
 # List of Docker-enabled make goals
 export DOCKER_MAKECMDGOALS_POSSIBLE = \
   all \
-  buildtest \
   buildtest-indocker \
   scan-build \
   scan-build-analyze \