docs/panfrost: Remove obsolete note on Android.mk

Android.mk was removed, so remove a reference to it from Panfrost
documentation. We should document building for Android, but it'll be
through meson. This note as-is adds more confusion than note.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!12914>
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