Remove unused rules for platform-robolectric-4.5.1-prebuilt

platform-robolectric-4.5.1-prebuilt was removed by
https://android-review.googlesource.com/c/platform/prebuilts/misc/+/2314082

,
and soong's special rules for platform-robolectric-4.5.1-prebuilt
is not needed anymore.

Test: m

Signed-off-by: utzcoz <utzcoz@gmail.com>
Change-Id: I953ec775505ccd0924f5a3f7fc29bdc7e573d335