Remove SOONG, SOONG_BUILD_NINJA, SOONG_IN_MAKE and SOONG_BOOTSTRAP.

These variables appear to be unused since Nougat, when Soong was
invoked through Make. This is no longer the case, since soong_build is
now invoked through soong_ui.

https://cs.android.com/android/platform/superproject/+/android-7.1.2_r36:build/core/soong.mk

Test: TH presubmit
Change-Id: I1cfe63144d559fa0da0e54474ed1e36fb3fa0555