.gitignore: there is no longer a built-in `git-rebase--interactive`

This went away in 0609b741a4 (rebase -i: combine rebase--interactive.c
with rebase.c, 2019-04-17).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -123,7 +123,6 @@
 /git-read-tree
 /git-rebase
 /git-rebase--common
-/git-rebase--interactive
 /git-rebase--preserve-merges
 /git-receive-pack
 /git-reflog