Remove unnecessary BUILD_DIR variable in Android CMake build (#27663)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27663
CMake sets CMAKE_BINARY_DIR and creates it automatically. Using this
allows us to use the -B command-line flag to CMake to specify an
alternate output directory.
Test Plan: Imported from OSS
Differential Revision: D18210316
Pulled By: dreiss
fbshipit-source-id: ba2f6bd4b881ddd00de73fe9c33d82645ad5495d
