tests: remove recipes that run tests with 'prove'

tests: remove recipes that run tests with 'prove'

Those recipes has served its purpose by helping us during the
development of TAP support in Automake, to ensure our TAP driver
didn't have gratuitous incompatibilities with the 'prove' test
runner.  But they are now unused, out-of-date and bitrotting.  So
it's better to just remove them.  We can resurrect them at a later
time if the need arises.

* tests/Makefile.am (PROVE, AM_PROVEFLAGS, AM_PROVECMD): Remove
these variables.
(prove, installprove): Remove these targets and their recipe.
(EXTRA_DIST): Don't distribute 'ax/prove-runner' anymore.
* ax/prove-runner: Delete.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>