rebase: remove a couple of redundant strategy tests

rebase: remove a couple of redundant strategy tests

Remove a test in t3402 that has been redundant ever since 80ff47957b
(rebase: remember strategy and strategy options, 2011-02-06).  That
commit added a new test, the first part of which (as noted in the old
commit message) duplicated an existing test.

Also remove a test t3418 that has been redundant since the merge backend
was removed in 68aa495b59 (rebase: implement --merge via the interactive
machinery, 2018-12-11), since it now tests the same code paths as the
preceding test.

Helped-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>