Remove more Android.mk files from build/make

soong_ui is passing the Android.mk file list to Kati, and it ignores
Android.mk files found via symlinks, so we don't need the workarounds
for build/make/target/board/Android.mk and build/target/board/Android.mk
both existing.

We've got dist support in Soong now, so we can remove the signapk
Android.mk

Bug: 122332221
Test: m dist out/dist/signapk.jar out/dist/libconscrypt_openjdk_jni.so
Change-Id: I2ac9016b04146340c5ac80f7090d00e186023574