Temporarily revert part of "tests: add a special setup [...]"

This conflicts with the `add-p` series and will be reapplied on top of
the merge later.

This reverts commit dc72b08.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

t/README

@@ -383,10 +383,6 @@ GIT_TEST_REBASE_USE_BUILTIN=<boolean>, when false, disables the
 builtin version of git-rebase. See 'rebase.useBuiltin' in
 git-config(1).
 
-GIT_TEST_STASH_USE_BUILTIN=<boolean>, when false, disables the
-built-in version of git-stash. See 'stash.useBuiltin' in
-git-config(1).
-
 GIT_TEST_INDEX_THREADS=<n> enables exercising the multi-threaded loading
 of the index for the whole test suite by bypassing the default number of
 cache entries and thread minimums. Setting this to 1 will make the