Delete __ANDROID_APEX_MIN_SDK_VERSION__ from Soong

Currently, Bazel sets __ANDROID_APEX_MIN_SDK_VERSION__ based on the cc library's min_sdk_version while Soong does based on apex's min_sdk_version. This causes a diff in the clang commands.

Digging further, we realized __ANDROID_APEX_MIN_SDK_VERSION__ isn't used anywhere so we might just delete it altogether instead of fixing it.

Bug: 265134933
Test: presubmit
Change-Id: I08e89dc8f6ef86669248ea5c47de6603e9d2ffbb