Remove deprecated experimental-new-pass-manager flag.

https://reviews.llvm.org/D136789 removed this flag, it's been a no-op
since 15.0.0. The current Android compiler is 15.0.3, but upcoming
compilers will error on the fact that this flag doesn't exist any more.

Change-Id: I54666175b521ed5cdd76c53ed700ad4ddf6931da
Test: make
Bug: N/A