autotools: Remove AC_CANONICAL_TARGET

We were adding the --target configure option in the mistaken
belief that it was useful for cross-compiling. In GNU terminology,
"target" refers to the architecture the programme being compiled
should itself target, as when building a cross-compiler.

To specify the platform the build itself should target one
instead uses --host, while --build refers to the system
where the build itself it taking place.

We were refering to the correct `host*` variables inside
configure.ac. The call to AC_CANONICAL_TARGET is simply
unnecessary.

Signed-off-by: Marvin Scholz <epirat07@gmail.com>
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passed #10454
docker
build

00:01:35