Remove external/pffft/.gitmodules and submodule directories. am: 2d0d66f988 am: 74e9dacf5c am: e23fdcccfd am: 5ec369b08b am: c02f030e19

Original change: https://android-review.googlesource.com/c/platform/external/pffft/+/1830698

Change-Id: I063484cbca50aef8d27bac49df0fb386bd376d24