builtin/{merge,rebase,revert}: remove GIT_TEST_MERGE_ALGORITHM

builtin/{merge,rebase,revert}: remove GIT_TEST_MERGE_ALGORITHM

This environment variable existed to allow the testsuite to reuse all
the merge-related tests in the testsuite while easily flipping between
the 'recursive' and the 'ort' backends.  Now that we have removed
merge-recursive and remapped 'recursive' to mean 'ort', we don't need
this scaffolding anymore.  Remove it from these three builtins.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>