refactor(make): better checks for tool existence and paths (#327)

After all the refactoring done so far, we can run checks directly
inside of `make`, because we have auto-cleanup, temporary environments
and we don't need wrapper scripts anymore.

Part of https://github.com/ooni/probe/issues/1466.

MOBILE/go

@@ -1,16 +0,0 @@
-#!/bin/sh
-
-# print the fullpath of the go we're using so that it's part
-# of the build logs and we can easily increase our confidence
-# that we are using the right go binary.
-
-echo -n "$0: checking for go... "
-go=`command -v go`
-if [ -z $go ]; then
-    echo "not found"
-    exit 1
-fi
-echo "$go"
-
-set -x
-$go "$@"

MOBILE/gomobile

@@ -1,24 +0,0 @@
-#!/bin/sh
-
-# print the fullpath of the go/gomobile we're using so that it's part
-# of the build logs and we can easily increase our confidence
-# that we are using the right go/gomobile binary.
-
-echo -n "$0: checking for go... "
-go=`command -v go`
-if [ -z $go ]; then
-    echo "not found"
-    exit 1
-fi
-echo "$go"
-
-echo -n "$0: checking for gomobile... "
-gomobile=`command -v gomobile`
-if [ -z $go ]; then
-    echo "not found"
-    exit 1
-fi
-echo "$gomobile"
-
-set -x
-$gomobile "$@"